The reason is your computer forgets its CMOS settings. During the boot-up process or POST (Power-On Self Test) a checksum is generated by the BIOS from the CMOS and compared to the one saved the last time the CMOS Setup was run or the BIOS defaults were loaded. If the two numbers don't agree it is an indication that the data in the CMOS has been corrupted. and a checksum error is issued by the BIOS.
A bad battery.
A battery that has become discharged (the computer has been off a very long time).
A disconnected battery.
Grounding the CMOS circuitry.
A bad motherboard.
A bad real-time clock.
A simple solution you can try:
change the CMOS battery.
Write down all of the settings from the various setup menus.
Power off your PC.
Open the case and locate the battery on the motherboard.
Obtain a replacement battery from a local or online computer parts dealer.
Remove the old battery and replace it with the new one.
Replace the case and power on the PC.
Enter your PC's setup mode.
Reenter the settings you have written down from the various setup menus.