Encountered a Unifi controller that would not load the web interface. I discovered that the drive was full and had to be expanded.
To expand a volume in AWS I had to go through the standard process of - Creating a snapshot, creating a volume from a snapshot, shutting the instance down, detaching, attaching (New volume), and booting the instance back up again.
The instance booted back up and checking df -h everything looked good! The only issue is while looking at the logs I found the following error- DbServer does not shutdown cleanly and need repairing on the next startup. Performing a clean reboot did not resolve the issue.
I was able to find the correct process for repairing the database for Debian Linux.
service unifi stop
mongod --dbpath /usr/lib/unifi/data/db --smallfiles --logpath /usr/lib/unifi/logs/server.log --repair
service unifi start