IMac hanging unresponsive during Yosemite 10.2.2 update

Question

This update began automatically while I was on vacation and has been hanging, with blank screen for hours. Although the hard drive seems to be running when I press the keyboard space bar, the machine is unresponsive to Command-L or any other input. Help, please!

Answer 1

Press the Power button down until the computer shuts off. Wait 10 seconds then reboot. If it's working then:
Repair the Hard Drive and Permissions - Yosemite, Mavericks, Mountain Lion or Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
Repair
When the recovery menu appears select Disk Utility and press the Continue button. After Disk Utility loads select the Macintosh HD entry from the the left side list.  Click on the First Aid tab, then click on the Repair Disk button. If Disk Utility reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit Disk Utility and return to the main menu. Select Restart from the Apple menu.
Try the update once more.