Call the place you got the money orders from, and explain the situation, and usually they will let you cash them. Good luck.
I can only suggest that you call the post office or talk with a bank and see if they can cash them for you sometime to do that though they will usually need the receipt in which you paid for the money order's a proof hope that helped you out a little bit.
You probably can't, which is why they tell you to hang on to the receipt. Is the check made out to someone else, or could you write it out to yourself? If it was returned by a company, ask them to issue their own check, since you lost your receipt. (Probably won't do it, though...) You may be out of luck. Go back to where you bought it, and ask them for advice.