The BIOS is stored in a solid state chip. What most people think of the BIOS is the CMOS, which is the program that lets you alter settings on the BIOS. That is what requires the battery backup. The BIOS is always there, and even if you remove the battery, will have default settings.
If nothing is coming up and you aren't even getting any beep codes, it sounds like the person introduced a static shock to the motherboard when they removed/replaced the battery, and the system is now dead.
With a little luck, you could find a replacement motherboard fairly cheap on ebay, and just switch over all the components. Just to a little reading on static electricity and how to properly ground yourself when working on a computer.
I have a Bx cel PC100 HT1543 motherboard that someone took out the battery for 15 min. It has a PII processor. The machine turns on, but nothing shows on the monitor. Is it possible to fix this - reload the bios somehow?