Sound file error when reopening project after saving and exiting

I imported a clip from YouTube and converted it to mp3 for a lip-sync project but when I saved and exited the project and came back later, it shows a tab saying sound file error and cannot attach sound file to audio clip. Also the editing I did to match up the lipsync with my animation did not save either. Is there any way to fix this error and bring back my old work or do I have to redo it?