Mould or other vegetation can smell whilst rotting down. As can dead rats/squirrels. I can only suggest emptying, cleaning, maybe using a sterilising solution, and then hope for more rain. Have to say I've not had that issue before either.
It's a mystery alright, it really stinks. I was hoping there might be some kind of cleaning solution I could put in it to break down any bacteria that might be causing the smell.
I have the same problem with mine Rocky. It's been ok for years but last year, OMG, the smell was awful and has remained so ever since. It's all silted up and I think I may need a new one, so I'd better take a trip to a local garden centre and price them. Good luck with yours. I don;t think anything got inside and died. I think it's just old stagnant water. Smells really foul though.
Avoid using chemicles, which may harm your plants.
Instead use a barley straw pouch, susspended in the water butt (like a tea bag), this should kill off the bacteria thats causing the nasty niff.
These pouches can be purchased from stores that sell ponds etc.
Just keep an eye on it Rocky.
The theory is, that the decaying process of the straw inhibits the type of bacteria that causes the bad smell.
Thats why I think its a good idea to tie a string to the pouch, so you can pull it up from time to time to examine it. eventually it will sink.