Old computers often does not work because of worn out backup battery on the motherboard. That battery keeps RTC, Real Time Clock running. Without it BIOS can not start operating system or at least there is some serious errors in startup.
In many cases that battery is not changeable. It is molded inside of RTC module. Module type is Dallas DS12887 or similar. One can buy new module from eBay on example, but it is NOS, New Old Stock because that part is obsolete. Batteries often worns out even without use in about ten years.
Solution is to remove old DS12887 module from motherboard and put own circuit in place.