Have you ever booted this motherboard successfully?
When I am not sure on which key, I will boot, and try the typical "ESC" "F2" "F8" "F9" "F10" "F12" "DEL" keys. But if it freezes, it does not sound like it is an issues of which key to press.
If this is a new build, and you have never gotten it to boot, I would say to reset the ram and if that does not work - it is probably an issue with the CPU (bent pins).