Try unplugging everything you do not need to post your system- just PSU, CPU, ram, VGA, power switch, and speaker. Unplug your reset switch even!
If no post then try removing the memory and VGA and see if you get a beep. If no beep with the memory and VGA out then you have a problem with the CPU, PSU, or MTB. Try floating the MTB outside of the case at this point, or try replacing one of those parts to troubleshoot which item is the issue. Also, you might try reseting the bios on the MTB as well.
Best of luck!