OP said bios update had finished, then the PC rebooted and that's when it tripped the mains fuse in fuse box. chances are the bios did update, but because the PSU failed on reboot to trip mains, we cant say for sure Mother board is toast.
you need a new PSU anyway, so before you get carried away taking out the motherboard for RMA I would try a new PSU first. after all you need a new one.
Not sure the Op could RMA the board. Might be worth a try but without the backed up file...
If the power went out before the update was written it's toast. That would happen at boot.