Library completely messed up - song files overwritten by other songs

Question

I am completely devastated. All of a sudden most of my song files are corrupt. When I click on a song to play it, it plays a different track in my library (as if the song is named incorrectly, which it isn't). Choosing to show the song file in the finder works fine, it shows the original file with the correct naming and correct file size.
But it isn't that song. It's a completely different song which starts wherever in the original song file. When the original song time ends the completely different song continues in the next original song file.
The original song files seem to randomly have been overwritten by other tracks in my library.
Same thing happens if I play the song in a different media player. Copying the file and adding it to a different computer/iTunes turns out the same way... although now it only includes the time and song title of the original track, artist and all other info is gone.
Should I go and kill myself? Or is there the slightest chance of a solution to this mess?
How could this happen? It's so weird.
The worst part of this is that I haven't been able to afford a large backup disk for my music files in quite some time. But for me, with iTunes, it's usually not the disk that has given up on my, it's been the library file. I've surpassed many library crashes but have never been near something as strange as this.
The disk seems to be running without problems or errors.

Answer 1

Thanks for your efforts in helping me out.
It is not a smart playlist. Standard.
Here are some screen shots to display the error.
1. File location when browsing for it in the finder - only two songs from the album available.
2. Songs in Music library - clicking upper left and searching upper right.
3. Regular playlist including all album tracks - playing one of the songs from the album.
4. File location (same as in 1.)- after choosing File -> Show in Finder