my pc black screened randomly

Define black screen? Any errors or it was more like the monitor blacked out? If the monitor, shine a flashlight into the screen, stick your nose right up and look to see if you can see a very faint image of your Windows desktop. If you can, your monitor's backlighting is failing.

Try another monitor.

well its a tv and i was watching a movie and then my screen tured black and sound stoped but the pc resumed but no signal or sound

How does it connect? Sounds like it could be a HDMI issue. I still recommend trying another monitor or TV and see what happens.

to connected with hdmi and the oly outer tv is mounted to the wall and will take years to take off

Trying another monitor/TV is the best and fastest way to determine if the problem is with the computer, or the TV.

Well it has not done it since

Let's hope it holds.