How and Why do computers CRASH?!?!?!

Heat & hardware compatibility also the reasons of crashes. My laptop crashes when i'm rendering a video or converting a video. And thats my problem.

"crash" is when you're doing something on your computer and all of a sudden, it shuts down. sometimes, viruses cause this crashes, which you can avoid by having updated anti-virus softwares. sometimes, it is caused by overusing your computer, when the computer is no longer able to withstand your usage practice, specially when you're low on memory resources. the processor can no longer properly process everything you've loaded, and crashes. c',)

Crash means to me that my computer doesn't work anymore. Not being very computer savvy, I don't really know (or care) what caused it to quit working. I just want it fixed!!!!!! The last time my computer crashed I had to get a new hard drive.