Welcome to the forums my friend!
Firstly what is your full system spec?
Is the DEBUG LED for your CPU lit up / on?
You should always clear away old paste - it shouldn't cause a CPU LED unless it was either causing a short or got onto another part of the CPU / socket (I.e. bins) for example.
I'd sooner be looking for bent pins and ensuring you plugged everything in correctly.
But cover this entire guide first to ensure you haven't missed anything obvious - and it will cover the bent pins issue:
https://forums.tomshardware.com/thr...ng-about-post-boot-no-video-problems.1285536/