So when was it last running? Any ideas as to why it stopped running? Hint: what changed?

Presumably SQL Server is running. Can you connect to any of the databases?

I had a similar experience recently (dev server thank goodness). Someone came around and said they couldn't connect to a DB. I tried logging in and got a message that the default DB couldn't be opened - disturbing as 'master' was the default db for that login. Found out the data drive had ran out of space. Freed up some disk space, restarted SQL Server, all go.