Do you remember anything important happening, before
this problem showed up ? How old is the computer ?
Do you turn off the power at the wall (remove the
AC) after each day of using the computer ? When
all power is removed, such as by flipping the switch
off on the back of the computer, that causes the
CMOS battery (CR2032) to be used to run the CMOS clock and
CMOS RAM, until the next day.