Boot the system from the installation media and use the "Repair your computer" link on the second page of the installer following the keyboard and language setup page, to get into the advanced troubleshooting options.
Often they seem to work better when employed from the installation media, try......
Startup Settings to reboot the system and then choose Safe Mode and or Safe Mode with Networking. Sometimes we see this issue when it is hung installing an update and booting into Safe Mode and then back to Normal Mode seems to kick something lose.
Failing that try the other Advanced Trouble shooting option
Startup Repair
System Restore