There are more clues you can get from more observation.
# Do you hear the fan running at all, or feel any breeze?
# If you have an HDD, when you turn it on do you hear the HDD doing its POST?
# Does the drive LED blink at all?
# Does it continue to blink?
1 = no, power failure
2 = no, HDD failure, power failure, MB failure.
3 = no, MB failure
4 = no, corrupt HDD, RAM, MB
If MB failure is suspected, start by feeling parts to see if any are stone cold or very hot.
If it is cold, it isn't working. Likely suspect would be the CPU.
Then remove RAM.
Then consider changing out the CPU.
Then consider buying a new computer. It gets uneconomical at this point.
A bad BIOS battery won't stop the computer from booting, but things will get strange and programs might not work.
A corrupt BIOS won't affect 1 or 2 above, but may affect 3 & 4.