Why does my computer hardware crash?

While using computer screen goes dark. Power light says lit. After several minutes I press-hold power button and after 5 seconds it powers down. I restart. It takes a very long time to boot up, then all is ok for months.

This has occurred a total of 3 times in the past 6 months.

I dont have any new apps or drivers installed that i know of.

I dont remember exactly what I was doing. It was either in my email app or reading something on the net or watching a video. I dont remember if it was triggered by an action on my part like a click or keystroke.

I didnt know if this is just an issue with monitor or with the actual computer.

So then I memorized keystrokes to power down the computer. I've been using those every time i turn off my computer since that 2nd occurrence above.

Months later while watching a video, it happened a 3rd time. I was 10 feet away and this long video is almost complete. The screen goes blank and this time the audio changes to a buzz-hum and does not stop. After a minute of no video and that constant buzz-hum I tried my shutdown keystrokes and that didnt work. After another minute i tried again. Nope, i cant shut down the computer. So, it's not just a monitor issue.

The next boot up took a long time. (maybe because of improper shutdown?)

That was several weeks ago and I'm still using the keystrokes to shut down my computer.

Ideas?