Q: MacBook Pro won't start after El Capitan upgrade

TThe second time i started my Mac after the upgrade it got stuck on the white screen with the Apple logo and the progress bar which does not load at all. I forced shutting down by pressing the off-button but the same thing happened when I started it again. i also tried the safe start by pressing shift after the starting sound but that didn't help either. does anyone have an idea how to fix this?