depends how far the bios got before the power went out. some older mb have a bios recovery in them if they still have some code on them. some will do it if a usb or cd with the flash rom is in the drive...you see at post the cd-rom or floppy light come on..
if not sometime there a keystoke to force a reflash.http://www.mydigitallife.info/recover-or-undo-corrupt-ami-bios-flash-update/
if not you have to contact the mb vendor to see if they can send you a replacement bios chip.