The computer starts but the screen will not render - it is Not black screen but bluish lines - keeps flickering but does...

Thanks. Tried this. Connected to external monitor(TV) with HDMI, used the F5 button. This renders both on laptop and external monitor. Rendered the same on both after some blue lines/delay- finally the Laptop did came up. So is it GPU/Mother Board issue? Is replacing motherboard a DIY. Is it even worth replacing? The laptop is 15”/i5/8GB/1TB/Win10 – it is 7-8 yrs old, had replaced the hard disk 3-4 yrs ago, and battery 3-4 months ago. It works absolutely fine when it comes up (whenever it does). I essentially use is as a backup machine. But it is becoming too unreliable even for a backup – random/long delays for startup are just too annoying. So what next?