Monitor and sound issues

Wondering if anyone here has the know how to help with a problem we are having with our desktop pc.

Lately for absolutely no reason, while the desktop is on, the monitor and the sound will just cut out completely and won't turn back on. The monitor just says "no signal to pc".

At first we thought it may have been a RAM issue as it seemed to only cut out when my brother would play minecraft on it, however that has now been ruled out as we have more than enough RAM to accommodate MInecraft and my brother hasn't played for a week and it is still doing it.

We thought maybe it was the monitor, that the cable was screwy, but that didn't explain why the sound would also cut out and we changed out the monitor with our other desktops monitor, but the same thing happened.

We have also ruled out any virus as we've done complete scans on the computer and there are no viruses.

Its getting frustrating as the problem seems to be getting worse. Now within 15 minutes of turning the computer on the signal to the monitor and speakers will just randomly cut out.

Anyone have any ideas?