one day it was playing them now it isnt what did i do or what happened?
They were probably corrupted, or something happened to your mp3 codec. Do the following:
Go into your Game folder, and then the Audio, and then BGM folder, and try playing your imported MP3s from there with Windows Media Player or RealPlayer...or whatever you've got. If they work, then you know it's not a problem with the files themselves. If they don't work then the files are corrupted and, well...my condolences.
If they're working outside RPG Maker, try re-importing them.
Did you change any of your game's scripts lately? That might be the problem. Otherwise, I can't think of anything else right now. :)