get yourself a cash tin and always stick the cash from your gigs in that. I did this when i kept on losing the cash from footie and it worked a treat, alot easier to find a big red box than a little slip of paper.

i know this wont help you now but just an idea

also you could maybe try speaking with the place you did the gig, ask them to cancel the cheque and reissue might take a week or two but at least you would get the money in the end