I just had the exact same thing happen to me yesterday. Any chance you've downloaded any new windows updates? That is the only thing I did before the crash. Bums!
Anyways, fear not. The music isn't lost. iTunes basically creates a library file that connects to all the music stored in your computer as sort of a directory. If the library file gets damaged, it can't connect to the music. No big deal, but it is time consuming. If you've recently made a backup of your library file, you can just use that. If not, I found the following worked for me:
1) Do not connect the iPod, first of all, until you're all done. That could sync the iPod with the computer and erase the music on the iPod. Major pain.
2) You've got to go into iTunes. Click on FILE and then "Add Folder to Library."
3) Find the folder that contains all your music. I am on Windows XP, so my path was C:\Users\Dave\Music. Basically iTunes will now re-index all the music within that folder and all the sub-folders so that they will be added to the iTunes Library file that it creates.
NOTE: You can also go into Windows Explorer and find the folder with all your music in it and just drag-and-drop it into the iTunes window.
4) Once you've got everything all loaded up, new playlists created (you will have to redo all your playlists, if you had any), make sure you save a copy of the Library as a backup. Either "FILE" "LIBRARY" "EXPORT LIBRARY" or "BACKUP TO DISC."
That is basically it. I did some housekeeping as well, as I had a ton of duplicate files, old stuff I never listened to ever, etc.
As a caveat, there may be other ways to do it, but this is all I could really find after a good deal of searching yesterday. With the cleanup, new install of iTunes 8.1, redoing all my playlists, the whole process took about 2 hours on and off.
Also, before step 2, I went into iTunes and selected everything that was showing (about 100 songs out of the 5000 that I had previously) and I deleted them. I just wanted a nice clean slate on which to work. When it asked me if I wanted to move the files to the recycle bin, I selected "Keep Files" so it only erased the info in iTunes, but not the actual music itself.
Good luck!