Maximus VI Formula has only one copy of BIOS. Some other models have two, which can be convenient in cases like this where a power interruption corrupted an update.
You may have to get with ASUS tech support on line and describe the problem as you did here, then ask whether a new, freshly programmed BIOS chip would solve it. You will probably have to buy the chip (wild guess $25). Then you can remove the board from the case, take off the ROG armor - plastic cover - by removing screws from the back. The BIOS chip is in a socket between the batter and the Z87 chip - pluck out the old and put in the new - no soldering involved. A lot of work, I know, but you'd have to do the worst of it to replace the board.
After the board is fixed, consider checking for updates manually every month or so rather than having a service prompt you. You can then download the raw file and use the USB flashback method which is more stable and secure than updating through Windows.
Jeff