The fact they played fine last week means nothing.
Look on the computer using Windows Explorer in the location where the files are supposed to be.
Your iTunes library files track the media you add to iTunes, how you've organized it, and other information such as playlists. By default, these two files are in your iTunes folder:
- Mac OS X: /Users/username/Music/iTunes/
- Windows XP: C:\Documents and Settings\username\My Documents\My Music\iTunes\
- Windows Vista: C:\Users\username\Music\iTunes\
- Windows 7: C:\Users\username\My Music\iTunes\
- Windows 8: C:\Users\username\My Music\iTunes\
Note: You may have an iTunes Music folder instead of an iTunes Media folder.