Almost all motherboards made in the past ~15 years use a CR2032
3V lithium battery to maintain the BIOS settings and clock. If you open the system up you should be able to see it on the motherboard somewhere. Carefully pop it out of the holder, and (after verifying the type of battery) buy a replacement of the same type and install it, making sure you orient it with the + and - sides facing the same way as the old one. You don't need the exact same brand as long as it is the same type (voltage and physical dimensions). Panasonic, Energizer, and Duracell are the most common "name brand" ones; I'd be a little leery of off-brand ones (they can already be half dead when you get them).
If you had made any tweaks to your BIOS settings prior to the problem starting, then those settings likely got reset to defaults when the battery died, so you'll need to apply them again.