Hi!
Does it just show a red dot? Or is there any pattern?
Anyway, most of the time it’s caused by the CPU/GPU. The solder under the IC’s is broken and this need to be fixed in order to get it working.
Here are some fixes you can try:
1: Reflow the CPU/GPU. This is the most easy and inexpensive fix to try. But most of the time temporary, so the issue can come back again.
2: Reball the solder. This is very difficult and requires special tools to get it done. But is very effective.
3: Replace the motherboard. Very easy to do so, but very expensive and hard to find.
If you need any guide or video, please let me know.