I read a few posts and most referred to Pram resets and SMC resets. This post worked though for me. It didn't work the first time and I "typed my whole username" instead of the first letter and it worked. It went straight to desktop and no "beachball" but it is working and for that, thanks.
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I had the same issue with my 2008 iMac and Apple support walked me through the fix yesterday...
1. Shut the computer down by holding the power button for 10 seconds
2. Restart the computer and press shift at the same time until you see the progress bar start moving
3. Once you reach the point where your screen goes dark and you see the cursor, type the first letter of the username for your computer, then hit Enter, then type your password, then hit Enter.
4. After a moment, you should see the spinning beach ball
5. Your screen should then move on to something along the lines of "Completing OSX Installation" (I can't remember the exact wording). Let it finish. After that it should go to your normal desktop and the issue should be fixed. (If your screen goes pitch black during this process, hit the space bar. I thought it wasn't working, but the screen was just sleeping).
Apparently it's an issue with an automatic update that never quite got past the login screen.
Hope this helps!"
This was on MacBook (13-inch, Late 2009). I updated today to 10.12.2 and it took several hours. Than a screen for linking a bluetooth keyboard came up with no inputs working. I had to initiate a hard shutdown and the above problem showed up. So something with the update than automatic restart failed today. Now the battery indicator shows as an "X" but it is still working under battery power and nothing happens when I attach the magsafe adaptor. The fan stays on continuously.
