Pc fix problem

I’ve just cleaned my pc and there are a few problems. Firstly my monitor says no signal, the HDMI cable works fine as I tested it on another device. And i was wondering if there is way to fix that. Also when i turn my computer on it works fine for a few seconds then turns off for a few seconds and then turns on again and repeats. Is there any way you know of fixing these problems or do i need to replace some parts