Well, try to boot to bios pressing delete (for desktop pc) and choose load setup defaults afterwards choose save & exit (F10 for ASUS Mobo)...
See if it works...
If it doesn't, you will have to open up the tower and find a jumper cap near the bottom-right of the mobo, take it out for 5 mins, then put it back in and power up the pc (this method totally resets the settings of the mobo), if you have the manual of your mobo, you could go to chapter Jumper -> Clear RTC RAM (CLRTC) it should explain the 2nd method much more detailed and probably with pictures...