PC displays no image to monitor. Not the monitor or the cable.

Why does my PC display no image, just a blank screen as though the monitor is not receiving any signal from the PC?

Recently my desktop was powering itself down. Its 6 years old, and had never been cleaned. I opened it up and it was beyond filthy. I removed all main components,I used a static mat and grounded myself to chassis while handling the components, cleaned the whole thing out carefully with compressed air, re-assembled and started it up. It worked, but In BIOS I checked the CPU temp and watched it climb about 1 degree every 5 seconds up into the 80s. Upon removing the side panel I could see that the CPU fan was not spinning. Assuming this was caused by the filth, and the likely cause of my PC's shutdowns I removed the heat sink and stock liquid cooler and radiator. I cleaned up all the old thermal compound carefully with rubbing alcohol and Q-tips as advised by dozens of instructional videos. I purchased a new heat sink and CPU fan, applied the thermal paste and installed the heat sink / fan as well as a new chassis fan. Plug it in and all lights come on and all fans are spinning. Connect to monitor and nothing. I remove heat sink, ensure that CPU is seated properly, re-clean and re-apply thermal paste, re-connect heat sink and CPU fan. Hook it up, no image. I order a new graphics card that is compatible with my motherboard, Install it. Hook up CPU, no image. During the handling I took all necessary safety precautions and ESD prevention measures. I've ruled out all of the easy fixes. It's not the monitor, it's not the cable, it's not the power supply. Now what do I do?
I'm currently in the final weeks of an A+ certification class at my local community college and the fact that I can't even troubleshoot my own Desktop is bothering me deeply. I know it's old and have since replaced the system with a new one, but I really wanted to get the old one working again to test my newly gained knowledge from this class. Any help will be greatly appreciated...