Perhaps this post needs more information...
I'm rebuilding a very old Windows 7 system. I reset the bios, which enabled the Realtek LAN that had previously been turned off. The LAN on the motherboard doesn't work (blew up in a power surge). When Windows loaded, it automatically installed the drivers. Somehow this caused my keyboard to no longer work in bios. Even disabling Realtek LAN in Windows didn't help. This led me to a CMOS reset to hopefully fix the problem.
The reset did fix the keyboard problem. I moved the jumpers to reset the bios. In the bios, I disabled the Realtek LAN again. One problem solved.
This system is more than 10 years old. I figured, while I'm in here why not replace the CMOS battery. I noticed my system clock falling pretty significantly behind (so much so I created a daily task to update the clock so as not to mess up Windows Media Center recordings).
I had to remove the heatsink to get to the battery. While removing the heatsink I noticed the thermal paste was a dried up cakey mess. So I cleaned the CPU and the heatsink and reapplied some way too old Arctic 5 I had laying around. The good news is that my system no runs about 30 degrees cooler than before.
So I replaced the CMOS battery and put everything back together. Turn it on -- no beep, no post, nothing. I make sure everything is in place and all cables connected. Still nothing.
On a whim, I put back in the old battery. And everything is back to normal. I know I put the new battery in the right way -- I double checked. I even tested the new battery with my multimeter. It's fine.
TL;DR... why would a brand new CMOS battery cause my system to not post?