You may have to reset the BIOS. Check your motherboard manual for the preferred method - it will also have an illustration to help you locate the reset jumper or battery. If you don't have the manual, most are downloadable from the PC or motherboard maker's website.
Short of that, power off AND UNPLUG the computer from the wall. Open the side panel and visually inspect the interior. If full of heat trapping dust, you will need to clean it. Locate the battery on the motherboard - it should be a typical CR2032 wafer battery, about the size of a quarter. Note the polarity before removing. Touch bare metal of the case BEFORE reaching in, then carefully pry out the battery with a non-metallic prying device taking EXTREME care not to slip, and scratch the motherboard.
Take the battery to your local battery/watch/camera counter - they will have a cross-reference list, and they usually will recycle the old one.
Do not touch the new battery with your bare skin. The oils attract dust and promote corrosion. I put a clean sock over my hand. When done, double check to ensure no wire were knocked loose. Upon first boot, go directly into the BIOS Setup Menu, as prompted. Set the date and time, and check to ensure all drives are identified and the boot devices setup. Be sure to "Save and Exit" to boot normally.
Keep us posted.