iTunes library songs all gone?

Suddenly for no reason at all my iTunes was magically transformed into "iTunes (damaged)"! x( Most songs are missing!

Now, I know this is not the first time this question was answered here. But I didn't find a fitting explanation. So I didn't do anything but just normally opened my iTunes, since it has so many songs that it takes a few days to listen to them all, it takes a while to load. I think, ok, this is normal and then suddenly I see a different loading window... it's not the usual one. It said something like "checking on iTunes library... songs not found/read/damaged." But before I could fully read what it said a new window popped up and said something like: "Since a few songs couldn't be found or read, the computer set a new iTunes called iTunes (damaged)."

And I click ok because I didn't know what happened! And then suddenly all my playlists and songs are gone! (Not completely, but like 90%). I didn't do nothing! I just recently downloaded two songs on my desktop from an E-mail. I didn't even open iTunes for them. Didn't even listen to them and now started iTunes seperately and this happened! What the hell is the cause of this? I might be freaking out insde more than others about this. But all the others, who had the same problem still have their songs on their iPod or somewhere. I don't. So it makes me really friggin mad.

I searched in my iTunes folders...in my iTunes music folder there seemed to be all the other songs (I don't know if that is complete), but when I opened some of them, the folders of the name of the songs were empty. Others were not. I would guess that even there are like 50% at least missing. But they are songs that are not on my iTunes anymore.

What I want to know is... WHY this happened. There was no reason, really. My computer crashed a few days ago and after that iTunes still was working just fine. But now.... -.-

And I want to know how I can fix this... if I need to do a system reset to a special date, then somebody tell me how I have to do this... cause I don't think I'd mind that.

Please, help! I would really appreciate it! Thank you so much for having the patience to help me and read this (in advance)!