If you're using a PC, I may be helpful...
Actually, it sounds like the reboot corrupted your iTunes Library. When iTunes restarted, it created a whole new iTunes Library. The good news is no files were lost. The bad news is all playlists and rankings were lost.
The first option to restore is that iTunes realized the error and created a file called:
iTunes Library (Damaged).itl Search for that, change the name to iTunes Library.itl and you should be good.
If the corruption happened because of a power surge or some other non-iTunes failure, you'll have to use a backup.
If you have a backup, all you have to do is close iTunes, delete the new files iTunes created, (sometimes I recommend reinstalling iTunes and before you allow iTunes to search for files, cancel) replace the new files with the old files (rename them if you have to). Restart iTunes and you're golden, save for any changes you've made since the last back up.
(everybody see how easy that was -
back up these two files immediately: iTunes Library.itl and
iTunes Music Library.xml - you can usually find them in the following directory: C:\Documents and Settings\[your username]\My Documents\My Music\iTunes)
Since I'm guessing you didn't create your own backups, when you upgraded to iTunes 7, sometimes iTunes makes a backup of your files for you. They can be found in the following directory:
C:\Documents and Settings\[your username]\My Documents\My Music\iTunes\Previous iTunes Libraries
If that's the case, you can usually copy them to the iTunes folder, rename them, and iTunes will use those files. Of course, this won't be a full restore, it will only be up until the day you upgraded. Also, if I can recommend something, do not use the orignal file to restore your library - use a copy in case something goes wrong on the restore so that you can try again. Sometimes the new 7.1 won't upgrade the 6.X library file automatically without a new install. And on rare occasiosn I've had to uninstall 7, install 6.x, use you old library, then re-upgrade to 7. You can get the old iTunes here:
http://www.apple.com/support/downloads/itunes605forwindows.html