Black Screen at Boot

Hello,

I am working on fixing up a computer from craigslist for a friend of mine. I replaced the HDD with an SSD and it was working fine. I ran AST and VST yesterday and both of them passed. Today I took out the logic board to reapply the thermal paste before I finished updating the computer and once I put it back together it won’t boot. I get the startup chime and then a black screen. I have checked the connections and they all seem to be seated well on the board and I made sure not the screw in the Processor and GPU’s heatsink too far. I’m out of ideas and if anyone else would have any that’s great! I am getting the correct boot chime as well as the ability to reset PRAM. I’m having some problems keeping the keyboard plugged in so when I get home from work I will try an SMC as well.

Any help is appreciated!

Thank you all in advance!