Hope you now know
DO NOT ALTER THE BIOS UNLESS YOU KNOW WHAT YOU ARE DOING
On the motherboard there is a jumper setting that will reset the CMOS to the default setting.
Should say CMOS Jumper it depend on the brand of motherboard
Normal should be Pin 1&2
Clear CMOS Pin 2&3
If you have a Manual of the motherboard check the setting or check the motherboard maker site before changing the jumper
Change jumper to clear the CMOS and wait 5 -10 mins then reinstall jumper to the original position