ABIT BH6 Main Board says CPU unworkable

When moved an ABIT BH6 Motherboard from one tower to another and upgraded the CPU to a 400MHZ Celeron, the motherboard said the CPU was unworkable. When I tried to use the old 266MHZ Celeron, it said the same thing. The motherboard would not let me into the BIOS Setup. Is there something that I can do to fix this?