It could be the graphics section of the mother board in which case an external card should overcome the problem ( plugs into the mother board with graphic wires to the pc
best consult a computer repair shop for a proper diagnosis and quote
be careful if they suggest a new pc as at most all you will need is a new mother board so seek other prices first
I had this pc built up in the existing tower and got a better deal than a pc off the shelf with a saving of around 40% so shop around as brand names still use components off the shelf that are lower quality that what you can get in a build up pc

I had that once but so long ago I can't exactly remember the cause.

I think I replaced the graphics card.
If my memory is correct I was sure it was the graphics card because my pc had on-board graphics that had been disabled because a graphics card had been fitted so I removed the card, transferred the video lead and restarted and it worked ok, just not as well. I replaced the graphics card and it was fine.

If your pc has on-board graphics that is currently being used, it probably means the motherboard is defunct, though fitting a graphics card in an expansion slot could render it usable once more.