Before anything, please note that I have found several similar questions on Stack Overflow and articles all over the web, but none of those helped me fix my issue:
This could be caused by the pid file created for postgres which has not been deleted due to unexpected shutdown. To fix this, remove this pid file.
Find the postgres data directory. On a MAC using homebrew it is /usr/local/var/postgres/, other systems it might be /usr/var/postgres/
Remove pid file by running:
rm postmaster.pid
Restart postgress. On Mac, run:
brew services restart postgresql