Sorry this isnt dog related but I know that some of you good people have cats and I am so distressed and looking for any kind of answers.
My 7 year old male cat (castrated) was seemingly well and fit last week , racing around the lawn like a kitten chasing butterflys, eating well , purring etc.  He went out on Saturday morning and still wasnt back by Sunday night.  I hadnt been unduly worried till then as he has always loved being outside and the only time he would stay inside for more than 10 minutes was if it was raining.  We are surrounded by fields and I guess he just loved being out there doing what cats do!  He finally returned on Monday afternoon and I was completely shocked as he was just skin and bone and when I picked him up he had a big hard lump in his abdomen. 
No sign of any injury at all.  He had something to eat and then cried to go outside again and sat in the shade in the garden.  By the time I had come off the phone to the vets he had dissapeared and I havent seen him since.
Now I cant think what on earth could have done that to him in such a short time.  I cant stop thinking that he may have been lying somewhere  since in pain and dying a slow lingering death and its just eating me up.  My only hope is that if something got him that quickly it killed him quickly as well.  Anyone any ideas what could have made him go down so fast? 
Thanks for any suggestions, I know it wont change anything but i just feel sick thinking about it.

he had a big hard lump in his abdomen. 

Did this feel like a foreign object he had swallowed >or< did it feel like it was a part of him growing inside?
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As far as I could tell it was growing inside him.  Couldn mess too much as he was obviously in discomfort.  Can cancer be that agressive and fast??? I feel so guilty that I didnt keep him in but as he was lying in the garden I though he would still be there when I got off the phone.
P.s. if it was a foreign object I wouldnt have though he would lose so much weight so quickly?

Can cancer be that agressive and fast???
I don't know in a small mammal >but< the tumor (sounds like) could have been there for longer than you knew but not highly visible when she was her normal weight & it was smaller, especially inconspicuos as we don't check our animals for such things. From what you write it sounds like it has grown rapidly, sorry, but it does not sound like a benign tumor.
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The only other thing I can suggest is that if you knew an approximate where she went and you know someone nearby with a dog ask them to put it on a lead and take it to the area, let it smell around naturally, on the lead, especially v good would be a spaniel but other dogs as well might focus on a scent.

Thank you. So many fields around us that he could be anywgere, but thanks for the suggestion.

I am sorry . Try putting a litter tray that he uses outside your door and also his bed and something with your smell on it as well.once he has returned a vet visit of course water and food especially  as it's so hot . Unfortunately it's hard to know what has happened until u and the vet can examine him but hopefully he will recover well. I would also alert neighbours etc that he is missing and too look in sheds garahes ,under hedges where he might hide if he is feeling unwell or to stay cooler.habe u rung the vets around to see if has been found and also put a notice in the vet . Is he chipped as that will help

Thank you!  I have been so upset that I didnt think of ringing local vets.  Years ago I had a cat that went missing and I did ring the vet and someone had taken him in after he had been run over.  Why didnt I remember that!  Thank you again for that!

Let us know how u get on .I have both cats and dogs and having one go missing is awful never mind possibly being unwell.ive also taken cats to the vet if I've found them as feel if the worst has happened at least knowing is better than always wandering.just to say that unless it's your.dog be careful taking a dog to look for.him .most cats will up and go pretty fast u less.they are unable too move if.the see or smell a dog. My flat coat has just flushed out a cat who was sleeping in the bushes that none of us saw  .the cat moved like.a streak of lightening and now I feel.bad that he was disturbed

Yes the worst thing is not knowing what has become of him.  I do have a dog and have taken him as far as I can bearing in mind how hot it is, but no sign.  Will let you know if I have any news, good or bad.

Yes do

The weight loss could be diabetes, but it's probably more to do with the lump.

I once had a young cat go out and never return. There's fields across from us, and a local dog walker suggested that he'd been got by a fox.
The not knowing is the worst.

Do hope you find him. The not knowing must be terrible. Thinking of you.

Update! Missing since Monday.

Sitting having my lunch today I heard a faint meow outside the back door and there he was.  It had obviously cost him a great deal of effort to get home as he was clearly exhausted.  I brought him indoors and he had something to eat but wouldnt drink.  I put him in the spare room while I got an appointment at the vets and gave him lots of fuss and some of his favourite treats.  I really had no expectation of him coming back from the vets.
However... The vet said he thought it was an abcess and after sedating him and shaving they found three tiny puncture wounds and indeed it was an abcess.
He is now home with AB's and painrelief.  One lucky cat, one very relieved me, oh and the dog is glad to have his friend back too!
Thanks to those who replied to my OP.

oh that is good news, hopefully he will be fit and healthy again in a few days , lucky he managed to get to the back door xxx

Im really glad to hear that, I'm also very glad it was not cancer, till I read you last post I was convinced that what it was.
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Such good news, I've been wondering. May he be back to normal soon.

How wonderful thank u for letting us know and may he have a speedy recovery and you a large glass of something suitable!

Brilliant news

So pleased he's home

Goldenfrenzy, great news but please try to keep the cat inside for at least a few days until he is recovered, cats that are unwell have a bad habit of going off and hiding away