|You should change the CMOS battery on motherboard, if this is a desktop PC ?|
CMOS battery retains the settings of the BIOS, together with time and date. If it is drained off, then other settings also default to the time your motherboard CMOS is made.
If you have made changes to the BIOS settings, then check them to see they have changed as well.
Most probably the battery is CR2032. But check the manual or see the battery on the motherboard in the PC case.