Posting this in case it helps someone else:
I was having this same problem as the OP after a hard reboot when my laptop crashed. What helped me was running the following command to see what PID was associated with postmaster.pid:
cat /usr/local/var/postgres/postmaster.pid
The first number that appears will be the PID. Looking in Activity Monitor, I was able to see that Postgres was running, but without a PID number that matched the one shown.
Instead of the steps outlined in the answer referenced on Superuser, I restarted my laptop properly and then opened up Terminal and ran
brew services restart postgresql
This worked without having to remove postmaster.pid, which I saw a few other posts recommend. Sometimes it’s the simple solutions that work.