lol, you did it via the battery - old school.
You can actually just short the CMOS jumper pins for 5 seconds (see page 18 in the manual).
Looking at the BIOS default settings - it seems that the PCI is set to default graphics output for some reason. However, I assume it will still output from the onboard graphics as it presumably worked in the past. If would try resetting the CMOS using the jumper method, ensure that there is nothing in any of the expansion ports (PCI and PCIe) and try and boot up. I suggest spamming "F2" to see if you can see the BIOS.
Also, can I confirm that both the 24pin and 4 pin power connectors from the PSU are installed correctly on the motherboard?