If the system worked fine before your maintenance, then something has changed which happened during that process. Even though you checked all the connections I would suggest checking each one carefully. Also make sure the CMOS battery is properly connected. Reseat the RAM and if need be disconnect and reconnect any cables you may have touched. The cooling fans plug in to the bottom of the board where you would not notice if they became unplugged. I would check them carefully as well.
Hello all  I don’t know what’s wrong with my laptop 7577, i7, 16gb ram in one slot second slot empty, 256gb ssd, 1tb hdd, 1060 6gb max q.I opened the laptop to re paste. However after putting it back together the laptop won’t start and no display. When I press power button the laptop ‘tries’ to start but within a few seconds it turns off - display remains off at this time. And if I let it be then it turns on again trying to start and turns off in a few seconds and this keeps repeating and no display so don’t know what’s happening. ( when I reassembled,  all screws were accounted for and I have double checked all the connectors are properly seated. )   i am not sure what’s happening with this so any help is much appreciated. 
7577 laptop won’t start