When you rip music (from audio cds) to your computer, the default location is "My Music" or "Music" folder. You didn't specify your operating sytem (XP, Vista, OSX, etc.), so I'm guessing here (I use Vista and sometimes XP). You can also do a search for iTunes Library (using "find the file"), but you may just get the database files rather than the actual music files. It's worth a shot though. You can also see where iTunes puts your music. In iTunes, go to the edit menu, then preferences, then the advanced tab and the general subtab. That will show you where iTunes puts your music when you rip cds or make purchases on iTunes. Then go to that location and see if you have your music there. If it's there, then you can drag it back into iTunes.
If you still can't find your music, I guess you can try looking in your recyle bin. Accidently deleted? Does anyone besides yourself use your computer? Have you noticed an increase in hard drive space available? You might want to have someone more computer savvy than yourself try looking for them.
If the music is gone, you might be out of luck. If you ripped music from your cds then you can go through the process of ripping them again. If you bought from iTunes, they usually don't let you redownload.
In any case, whether your music is still on your computer or you ended up losing it and having to build it again, you really ought to back up your music and any other data that you can't afford to lose. I have another pc, a laptop, two external hard drives, and I burn my purchased music to a data disc dvd. You don't necessarily have to go to those extremes, but probably at least an external hard drive and additionally a data disc for iTunes purchases.
The i-tunes library shows the entire list of my music but when I click on a song, the pop up message states that the file cannot be found. Clicking the "find the file" comes up with nothing. I don't know how to find the original files on my PC. Are all of the songs that I have downloaded on this pc now corrupted and gone? I know next to nothing of computers, so I need assistance. HELP!