|Replace the CMOS battery. It is located on the motherboard. Look for a silver coin shaped battery. Take care to not which side goes up before removing it.|
To get things working in the mean time boot into the BIOS (setup) screens and reset the date & time. Possibly the boot order too.
If you keep power to the computer the settings will hold. Using a power strip or unplugging the computer will result in the settings being lost again. The battery is there only to hold those settings when there is no power to the computer.