Replace the battery, 6 bucks at most at a Target, Walgreens, Bestbuy, or any local store that sells watch batteries.
And for those who don't know CMOS, it is the function of your motherboard that controls date and time as a function but also stores memory of the motherboard aka BIOS settings. Dead battery means well, nothing terribly good, that is if you want your settings saved properly.
Edit: When replacing the battery, do the swap as fast as possible to avoid loss of data but considering it's dead for a bit now, doesn't matter.