I had made some config changes to the database earlier in connection with the move to a new server, that caused the storage usage of the database to grow a lot. Then when it had no more space left, it crashed which caused the downtime. Unfortunately it happened at a time, where I wasn't available to fix immediately, which is why it was down for so long.

It is now fixed and I will keep a watch(setup an alert for database disk usage) to make sure it doesn't happen again.