Hi newcomer and welcome to the Tom's hardware forum.
Ur question isn't clear.
1- U say that change the mobo BUT at the same time u flash the BIOS.
2- What mobo and CPU do u have?
3- Like I see exist two options for ur problem. a) Ur flash BIOS wasn't good and the CPU that u have isn't supported by ur BIOS version. b) U did a bad flash BIOS and kill ur mobo