Dell Optiplex GX520 won't restart after cleaning

Here's my problem.

Computer was operating normal, but the fan was starting to sound a little loud. Disconnected, removed the heatsink, blew the dust out, cleaned off the old thermal paste. I did not have new paste, so I reassembled without. Computer restarted and ran normally. It operated with no problems, and after a week I finally obtained some new paste. 

Disconnected, removed the heatsink, followed the instructions for the new Arctic Silver thermal paste, and reassembled. Hit the start button, fan starts, no monitor, beep code of 1 - 3 - 2.

The computer is a refurbished unit upgraded from XP to Windows 7. Intel Pentium 4, CPU 3.2 , 1 core. The BIOS has not been updated, is A05.

I just believe it's something simple, but I'm lost.

Help