How long has it been since you looked in your garden shed Karen? Has it been locked in and died or can animals access it in and out?
If it has been locked in and died I would take JG's option,
If it could get out, but has died anyway then I would get the authorities to look at it, it may have died from something unsavory, or it may be old age or have been hurt and crawled in there to die, but I guess it would be best to be investigate.