Did you leave batteries in it? If so have they leaked everywhere?
Kick52 might be right there, clean everything out as well as you can including taking it apart and reassembling it. Check that the battery connections to the board are are still in place. If you are running it from a adapter try bypassing the power plug and connecting the power supply to the board with crocodile clips or something to see if the socket is dead.
One thing you should look out for on 303's and 606's after they have been left around for a while is tiny cracks in the board causing gaps in solder tracks or broken joints. The circuitboards on the 303 and 606 are just cheap rubbish and often shrink over time causing these problems. Its usually easy to fix but finding the problem in the first place can be difficult.
Theres any number of problems that develop with 606's as realistically they were never built to last. If you can't see anything obvious then i wouldn't try messing with it if you don't know what you are doing.