Why does my computer not boot?

You should not have connected anything not need for the system to POST until you confirmed that the system does POST. And you should not have connected anything not needed for the system to complete a basic hardware test (like memtest86+) until such a test was completed. You're making more work for yourself by adding too many variables.

Disconnect everything not needed for the machine to post. Leave connected only one RAM stick, the video card, CPU, and power supply. See if you get a beep and display. If not, clear the CMOS.

Did anything go wrong or not seem quite right when you put the machine back together? Did you remember to connect both power cables to the motherboard? Another thing that could cause this is leaving the clear CMOS jumper in the "clear" position.