Sudden restart, blue screen

Question

There I was, sitting there, minding my own business when suddenly my screen turned blue; all that was visible other than blue-ness was the clock int he upper-left corner of the screen; a minute later, I was prompted for my username and password like I always am when I log in.
What causes this? How can I prevent it? What should I look for?
I have been running this machine with the same installation of Tiger since I bought it 18 months ago. I had my first kernel panic two weeks ago, last week several open applications all crashed at the same time.
Thanks
P.S.: Pardon me for the tone here if I am being inappropriate in some way, this is my first time posting here.
G5   Mac OS X (10.4.9)  

Answer 1

I know that. Considering that this is a single-user system (i.e., I am the only person who uses this computer, nobody remotely logs into it either), no sysadmin involuntarily logged me out.
It would be so helpful if someone could tell me what could cause this to happen, however. Perhaps there is a log file somewhere that I could look at and identify the problem. This is what I am asking, for the cause, not for a definition of what the event is.
G5 Mac OS X (10.4.9)