We saw the following error a few times popping up after we have configured SharePoint 2013.
The Managed Metadata Service or connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started.
This error is connected to the following error in the Event Viewer.
Event ID 8088
The Managed Metadata Service ‘Managed Metadata Service Application’ is inaccessible.
Quick solutions
There are a few things you can check to see if this issue is easily solved
1. Restart the Managed Metadata Web Service
2. Verify permissions for the service account on the Service Application
Long solution
I could not fix this issue with additional permissions or by restarting the service/server and I have re-created the service application but kept the database.
1. Log on to the SQL Server Management Studio
2. Backup the Managed Metadata Database
3. Log on to the SharePoint 2013 Central Administration
4. Delete the Managed Metadata Service Application (Do not delete data associated with the service application)
5. Recreate the Managed Metadata Service Application using the Central Administration. (Create a temporary database)
6. Verify the new Managed Metadata Service Application is operational
7. Edit the properties of the MMSA
8. Rename the Database name in properties to the old database
9. Perform an IISReset
10. Verify if the terms are back in the MMSA
11. Delete the temporary database
12. Delete the backup file
