At one of our customers we had the error message, that the managed metadata service or the connection is not available. I am not sure, but it may be due to an update we installed a few days earlier.
I read a post which says that the application pool account does not have enough rights to read the registry. The suggestion was to run the following powershell script:
psconfig -cmd secureresources
After i did this, it starts a script similiar to the configuration wizard with three steps and after that we did an issreset. It was working fine after doing this. Hope this helps you some day.