Did you follow the instructions provided by the Asrock site?
Which method (Windows, DOS, Instant Flash through the UEFI menu) did you use and did the update complete successfully?
Does it appear to be booting (initialising disks etc.) but without video output, or does it halt on power on?
Have you tried with one video card connected?
What LED status code is shown on the "Dr Debug" indicator (located near the SATA ports)?
As per Asrock's warning, unfortunately it is possible for BIOS updates to go wrong leaving the machine unbootable. It's rare (I've done hundreds of BIOS updates, on many different machines over the years without bricking any) but in those rare cases only the manufacturers are likely to be able to fix it.