To recover accidentally deleted files or files that simply gone missing by getting deleted, i would recommend Recuva.
I have personally used it a lot of times with great success, whether from hard drives or flash drives.
From their website its description is:
Accidentally deleted an important file? Lost something important when your computer crashed? No problem! Recuva recovers files deleted from your Windows computer, Recycle Bin, digital camera card, or MP3 player. And it's free!