iTunes can't handle files being moved around outside of iTunes on a Windows PC.
The paths to the actual music files are stored in the ITL file:
http://support.apple.com/kb/HT1660
Do you say you moved everything back to the original location on the C: drive, but iTunes still won't "see" them?
You can check to see if a Previous Itunes Library file works OK.
Press *and hold* the shift button while opening itunes, and choose an ITL file in the Previous iTunes Library folder.
What you need to do in the long run, is
consolidate your library onto the external drive. Changing the location of the itunes music folder only affects
future additions to the library.