If you can’t boot after a BIOS update, it’s probably because of one of these reasons (I’ll try to put them in order of most likely to least likely):
1. The BIOS update didn’t keep your BIOS customizations or implemented new customizations for your default boot device. To fix this, change your boot order to boot from the drive containing your operating system.
2. The update didn’t keep your BIOS customizations or implemented new customizations for your hard drive configuration – it may have changed the AHCI emulation to RAID or vice versa. Alternatively, it could have changed your RAID configuration. If your BIOS has such settings, set them back to the correct way or restore your RAID settings if required.
3. The update didn’t keep your BIOS customizations or implemented new customizations pertaining to Secure Boot mode vs. Legacy Boot mode. Try switching between the two to see if you can make it boot.
4. The BIOS update wiped your Secure Boot Keys. If you have a backup of your keys, restore it. If not, you may need to reinstall or restore your operating system (the good news is that your backup software will likely still have access to your hard drive, so you can back it up then restore it without losing anything).
5. The BIOS hasn’t detected your hard drives. Ensure they’re detectable.
Finally, if all the above fails, you can try reverting to the previous BIOS or installing a different BIOS version.
I should probably say that the above suggestions assume that you can’t boot at all, rather than the operating system being booted into then failing to start. An error you might get after updating the BIOS might say something along the lines of:
Unable to find operating system on this disk.
If your computer starts to boot into Windows then fails, that’s probably a different probably.