I've had two days of nightmarish problems with the same unit. I had to take it apart to remove the CMOS battery in order to set the BIOS default settings as I could not enter the BIOS either, nor did my display work (via DisplayPort) so there were no error messages to guide me.
Try the following (assuming your keyboard works - this is another inherent problem with Fastboot enabled):
If your unit does not reboot, hold the power button down for a couple of seconds or pull the AC plug on the unit while on. You should be able to enter the BIOS upon turning on the unit the next time. If this fails to work, then I cannot help on this matter.
Thank you for your comment, I will try that when I get home from work. You think it is because the Fastboot feature has been enabled after the BIOS upgrade? I didn't have it enabled before.
imwithid Thank you very much for your help. Your suggestion solved my problem right away.
I have since experimented with the BIOS settings and discovered that the problem appears as soon as I disable Legacy boot (actually I enabled Secure boot and then it disabled Legacy boot by itself. But disabling both secure boot and Legacy boot still gave me the problem so my conclusion is that Legacy boot must be enabled if I want to be able to see the BIOS. I can't see any difference in boot times so I don't really care.
Again, thank you for your help.
You are very welcome. I'm glad this solved your problem.