If neither of these worked, then you will need to remove all installed cards and disconnect all connected drives/devices on the board. Leave only the CPU, its fan and a stick of ram installed. Connect video to onboard if equipped. Please perform the CLR_CMOS procedure again, disconnecting power and pulling the battery for 5 min.
Re-insert the battery and attempt to start the system, reconnect power and attempt to start again and enter BIOS. With no devices connected, you should get the Gigabyte Logo and have the ability to press <Del> to enter BIOS.
If this doesn't resolve the issue, the only things left are:
-Attempt to invoke the back up BIOS
-Attempt to re-flash the BIOS (not sure if this is possible on an older board if you can't see the screen)
-Remove the board from the case and perform further bench testing
The behavior black screen after a failed install or upgrade attempt is often seen when the system cannot find a boot device, or when it cannot be read. Normally this times out (after attempting all items in the boot order) and the system displays a boot error message of some sort.
Not in your case however, so the next step in to reset the BIOS. When this fails, you remove devices, reset again and attempt to start. Failing again, with the older boards that don't have BIOS switches, you attempt to invoke boot from the back up BIOS. If you get a loop and you know the hardware is good, no bent pins in socket, etc, we have seen cases where forcing the primary and secondary BIOS to re-sync or flash using ALT+F10 or ALT+F12 depending on Award or AMI.... has fixed things... but you aren't looping and cannot get display.
So you need to remove as many variables as possible and attempt to get a clean start. I think I've pretty much covered the basics. If anyone else has something to add, please feel free.