HELP! No Display after cleaning out GPU and CPU fans.

I recently decided to take out the CPU and GPU to clean the fans and the inside of my computer but after I re-applied new thermal paste and put everything back the way it was, my display stopped showing. I did several things such as reseting the BIOS, swapping out RAM, taking the plugs out and re-plugging them back in. I then realized that there's this noise coming from the GPU as if it didn't have enough power to start or something. (Imagine the sound of something starting up then powering down and repeat). I took out the GPU and the noise went away. But my motherboard doesn't have integrated graphics so I didn't have a display anyways. Somebody please shed some light on this, any help is appreciated!