Because bad cables and monitors don't cause boot loops.how is that related to his problem!
BEFORE YOU OPEN THE CASE MAKE SURE PC IS UNPLUGGED FROM POWER OUTLET wait couple minutes before touching anything inside the case.if you see lights on motherboard try different monitors and different cables connect only to gpu .
CPU mounting damage or something caused by rough handling could
I'm just trying to root out all possibilities. OP is planning to rebuild the PC anyway.