Put it simply, CMOS is the onboard memory backed up by the small battery that keeps your BIOS settings.
If it gets corrupted, it can stop a clean boot, so clearing it by either disconnecting from mains and pulling the battery or near to the battery will be a set of pins to short. Then leave for 15mins.
On next boot the board will have been reset to factory defaults and you can go back in to BIOS and change what you like and save it.
Before doing that, I like to disconnect all the drives and leave a single stick or ram in to see it it can get to the missing boot device prompt and work from there.