I recently tried to upgrade my Mac Pro 2010 to High Sierra, 10.13.5. Everything looked to go OK - firmware update looked as if it applied OK, but after first restart - nothing. I can't even get the boot chime, or access the boot options screen. Tried resetting the SMC / PRAM etc but without success. Just a blank screen. The computer was working just fine up until the upgrade - any suggestions as to what the problem could be? Desperately need some help and guidance.