The BIOS (basic input/output system) is now normally stored in flash RAM, allowing updating by "flashing", unlike older types which were ROM or EPROM. However, the variables - clock, date, boot sequence, onboard video or video card etc are stored in CMOS which is maintained by the 'coin cell' battery, commonly 2032 - if this is old or there is a fault on the motherboard which discharges it then the variable settings will be lost. Simply replace the battery, power the system up and enter the BIOS when booting - a message appears briefly- usually hit Del or F1 - during the boot sequence, and set the variables. Then SAVE the changes and exit.
Good luck
MikeD