External monitor filled with static, blue squares, and red dots

My family bought a Mac like 8 or so years ago. I am unsure of the exact model, but says iMac on the back (Which isn't very specific, so I apologise).

You'd expect problems after a few years, so when our mac became cluttered with multi-coloured lines, I wasn't surprised.

Eventually, it got to be a problem when the screen started to have larger lines whose lights seemed to be failing. It got to a point in which the whole screen had static, and there was very little of the screen that could actually be scene. It was easy enough to fix, we just got a new monitor.

It did the job, no more lines, the screen did have a yellow tint, but I wasn't worried.

Now, I was watching some youtube videos, then all of a sudden, both screens stopped, and red dots appeared. After countless efforts to exit the video, I unplugged the computer. When I turned it back on, it was even worse, with blue squares filling the screen, static filling it, and countless more red dots.

While the external monitor does not have the multicoloured lines, and such, both seem to have the same blue squares, static and red dots, at least for what I can tell.

Curiously enough, the waiting symbol for the cursor appeared and none of the screen problems overlapped the original.

Did I do something to it?

Is there some way to fix it, or will I have to get a new one?

Also, I apologise if this question is in the wrong category, I'm new.