After one of the latest updates, that started to happen to me too. Interestingly, that thing happens only after a power outage, where the Raspberry Pi didn't get shut down properly – every time. With previous versions, that did not happen, even when power was unstable.
Naturally, when that happens, the http://pi.hole/ domain does not resolve too.
However, I do not have to reinstall; running 'pihole -r' and selecting "repair" fixes the problem. Yet, it's a bit of a pain to do that every time the power goes down, and it has been unstable here for the last couple of days.
Perhaps some file gets corrupted?
Thanks!
UPDATE: Even rebooting Raspbian properly has the same consequences, requiring a repair ('pihole -r'). Perhaps some file does not get properly closed, causing corruption?
if you try to run :
pihole -g
as root in the console, I think is going to work.
the issue is because the user runing the web server that running the pihole admin interface do not have right on the file /etc/pihole/gravity.list
I use facl to add the required rights and everithing is working well now.