I'm a non-expert PC user. I am using a Lenovo AIO 700-24ISH Ideacentre desktop PC. The OS is Windows 10 Home. There has not been physical damage to the PC before the fault. Windows Defence Security Centre does not report any known problems.
I have uploaded a video showing how the problem looks like on youtube at https://youtu.be/R8RIPdgAaVk.
I'd be grateful for any help and guidance.