The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started. Please Contact your Administrator.

Root Cause:
 Your app pool might have stopped.
Or Manage metadata service has not created in CA.

Solution: Start AppPool.
Or
Go to SharePoint Central Administration
click on Manage service applications
Click On "New"
Click on "Manage Metadata Service"
Enter all the details.

Save

Now try Managed Metadata Service