Which mobo? Take out the CMOS battery from the mobo. Keep it out for a while and then plug it again, it will clear CMOS and you will be able to get into BIOS on next startup.
There is another way around: Press and on hold on F2 and power on the system, this will take you into BIOS
From a powered off state, press and hold the power button for about 2 seconds, release it when you hear beeps. (Not all mobos support this)
Good Luck