Finally found a solution to this after about 2 weeks of on and off troubleshooting. This was in Windows 8.1. Indexing was not running, all options were grayed out, and no solutions I tried (registry changes, etc) worked.
A few posters suggested going to 2 directories and deleting all the files in those folders:
C:\ProgramData\Microsoft\Search\Data\Temp\
C:\ProgramData\Microsoft\Search\Data\Applications\Windows\
However, this solution was initially impossible for me, because the first directory (Temp) and all the files in the 2nd directory (Windows) would DISAPPEAR upon trying to view them. They would be there for a second when I first entered the directory, and then vanish. Since I couldn't get to them, I couldn't delete them. I've never seen files and folders do that before.
I fixed this by renaming the "Windows" folder to "Windows.old," and immediately the system generated a replacement where the files didn't disappear! I then did the same thing with the "Temp" folder, but this time it didn't auto-generate a new folder, so I just made one myself.
I didn't even have to delete the files in the newly generated folders. Indexing started working immediately after the new folders were created.
Hope this helps someone!