Hi Ramesh,
BIOS upgarde will require the server to reboot.The firmware bundle, which includes BIOS, when updated requires the blade server to reboot This will affect any running VM's on that server.It wont change the config of ESX however.
If the server is taking some values from a pre defined pool, for example like UUID pool, and this pool gets exhausted before the server boots up then there will be no uuid allocated to the server so it is imp to make sure that all pools have sufficient free resources.
Thanks and Regards,
Geevarghese