Years ago I lost my entire collection, outside of what I had on an 80gb iPod. What happened was this:
(Let me preface this by saying I had (and still have) a 4 bay Firewire HD box, that, at that time, held 4 250gb drives. I had them set up as a raid, but, like an idiot, had it set up as a Raid 0, which, while good at the time (1tb of storage space, in one large chunk), made it impossible for me to recover anything (no backup))
I had (and still have a Mac). This is when Boot Camp was first released, which allowed you to dual boot your Mac, and install & run Windows natively. I hate Windows, but like most ppl, I was curious how it would work on my Mac.
Like an idiot, I left everything connected to my CPU when Windows was installing. All of a sudden, I see "formatting driveS" pop up, and looked at my external box, and the activity lights for each bay were blinking. I stared at the screen for about 10 seconds before I came to my senses and canceled the process. Too late.
I was just sitting there staring at the screen, and couldn't say shyt. My girl asked me what happened, and I couldn't answer. To this day, that is the maddest I've ever been in my entire life. My pops is an IT guy, so after I tried some half assed recovery programs, I brought my drives over there to see if they could be recovered. They couldn't. I lost probably around 3-400gb of music, and another 200gb of movies/porn.
It was kind of a blessing to me though. My collection was a gigantic mess: shytty quality of stuff, horrible tagging jobs on my entire library and not very much album artwork added. Starting from scratch (using the stuff on my 80gb iPod as a starting point), I was able to make sure that my library is clean. I let iTunes manage my music now, and I was able to fix any tagging issues right as I got the music. I also stay far away from shytty quality rips.
Also, I learned a hard lesson (after completely ignoring my Dad's advice when he got me the HD bay): BACK. YOUR. STUFF. UP. Since that day, I've always had my music backed up. And now, I have a Drobo, which does it automatically, so I can sleep peacefully.