Tried updatIng from Sierra to High Sierra last night on my 13” 2015 MBP. Downloaded from App Store, then went to bed and left installing overnight. Stupidly I didn’t do a back up. Got up this morning to find fan running a the same black screen that I left ten hours earlier. Nothing would work. Powered off and tried to restart using CMD+ R, but nothing happens. I cannot even start the computer. Any ideas or suggestions as to what I might try next please?