Sorry to stir the gloomy pot, but recovering from a flat CMOS battery can be a right pig.
Sometimes hard to change them without removing and replacing connectors, which can add more faults. When the battery dies it can leave the BIOS in a distressed state, and so does simply plugging in a fresh battery. Usually necessary to short the BIOS out to reset it, for which purpose a jumper may be provided. Or not! If not, remove the old battery and leave well alone for between 10 to 90 minutes, then put the new battery in.
Some machines come with the battery soldered in; it’s not expected to last longer than the battery! Others are fitted with super capacitors rather than batteries, causing other peculiarities!
How to get into the BIOS varies between makers and models, and breaking in can be tricky. Not always clear once inside what needs to be done to restore order.
If simply changing the battery and jumpering the BIOS doesn’t fix it, I suggest taking it to a repair shop. They’ve seen it all!
At work, dealing with desktops and laptops in 6 figure numbers, a percentage were written-off due to old-age battery failure. Repairs cost money and machines old enough to suffer battery failure are likely to fail again for other reasons. Not worth fixing. If it wasn’t a cheap, quick fix, default action was to scrap it. The organisation got a faster computer that was good for another 7 to 10 years. (Laptops didn’t last so long – people drop them!) Though budget managers hoped they would last forever, no-one expected computers would.
Whether or not a new computer will run ancient software is another nasty question… Is the model available second-hand?
Dave