I updated my iMac 2011 to High Sierra from Yosemite.

I updated my iMac 2011 to High Sierra. It was installed, then said it needed to restart. Up popped several dialog boxes saying that several extensions had been installed incorrectly. High Sierra said there was an error. I clicked OK and tried to restart as usual from the Apple menu. Now I have a black screen. I tried using recovery mode. It was almost booted when the black screen came back up. Any suggestions?