Display won't work after cleaning

My son's ps4 was overheating. It would make a loud noise and then power down. I followed one of your videos and took it apart cleaned everything. The heat sink was jammed packed with dust. I cleaned and replaced the thermal paste and put it back together. I plugged it in and it turned on and sounded like it was running really smooth. I could hear the fan going but now the display won't work. My tv says no signal. It worked before I started. I used a flash light to check the hdmi port and theft are no bent or broken prongs. I took back apart and re watched the video and put it back together again and have the same problem. What did I do wrong?