I had upgraded my Ubuntu 16.04 to Ubuntu 18.04. The upgrade completed and the system tried rebooting, but after that my PC no longer boots. I get a black screen, with no disk activity. If I press the power button my PC turns off immediately (no need for long press). I tried Boot-Repair but the process completes with errors. I think the last one is the most relevant.
I fixed it by disabling SecureBoot from UEFI. I don't know why after the update this had become a problem (since with Ubuntu 16.04 SecureBoot has always remained activated) and currently I do not have too much time to investigate.
In any case, thanks to GabrielaGarcia for the clarifications.