Ok this seemed to help:
Try this command,
sudo service mysql start
My database seems to be up and Ghost instance is running again (but there seems to be a NGINX problem still).
Edit: Ok both my sites are now back up! I changed the port of the second site to 2369, using:
ghost config server.port 2369
The command in the StackOverflow post helped, and I will also link this topic as I used it when I first got Ghost up and running according to the article:
And I have managed to successfully update to Ghost v4