Depending on the make and model of the mainboard, the firmware can be usually be reinstalled by using a device that allows for in-circuit reprogramming. A MicroChip brand CPU uses an interface called ICSP. Some Atmega processors have a clamp-on-the -CPU adapter to allow the flash on board to be reprogrammed. It is like a reverse socket that makes a connection to the pins on the processor while mounted on the board. Other CPUs have similar capabilities.
Find a University or Technical College or Community College electronics instructor and explain your problem. If they do not know who can do it or how to do it, they will likely know who to refer you to. Sometimes a graduate student in Electrical or Computer Engineering will have all the tools at their disposal and can show you how or for a fee, get your firmware reinstalled.