So, Any ideas to computer crash?

Does anyone have any inkling what could possibly have gone wrong with my oldie computer? It was running really slow so I was going to defragment it, only when I started to do it I got an error message telling me it could not be done because there were disk errors or corrupt files (can't remember which). So I told the 'puter to fix it when it asked. I forgot to run the defragmentor after that, but everything seemed fine---was running faster than normal.
I used the scanner Friday night and left the computer on afterwards. When I went to use it the next morning I had a blue screen with a fatal exception error. This has happened a couple times over the years and I just reboot and it seems fine. Now however, it will not reboot. I can't get it to start up. The powersaver sleep light remains lit when normally it flickers when the system is starting up. It's a Compaq Presario 4550 (probably 8-9 years old) and I have never had any problem with it before. I plan on looking for something new, but am just wondering if anyone knows what it could be and if its salvageable where I could give it to a family member if it wouldn't cost much to fix.

Thanks much!
Sandy