Sometimes you run into an error message that can mean any one of several things has gone wrong, and there's no real way to know what is the true solution. The error "Managed Metadata Service or Connection is currently not available" in SharePoint is one of those error messages. As an example, look at this thread - there are multiple solutions, each which seems to have worked for different users.
I encountered this error, and the error in the ULS Log was:
11/23/2016 11:32:17.42w3wp.exe (0x152C)0x1D78SharePoint ServerTaxonomyf8bnMonitorableFailed to get term store for proxy 'XXX Managed Metadata'. Exception: System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at Microsoft.SharePoint.Taxonomy.Internal.XmlDataReader.GetDateTime(String name)
at Microsoft.SharePoint.Taxonomy.Internal.SharedTermStore.DeserializeLastChangeTime(IDataReader dataReader)
at Microsoft.SharePoint.Taxonomy.Internal.SharedTermStore.Initialize(IDataReader dataReader, Guid termStoreIdValue, Boolean fromPersistedData)
at Microsoft.SharePoint.Taxonomy.Internal.DataAccessManager.GetTermStoreData(MetadataWebServiceApplicationProxy sharedServiceProxy, Boolean& partitionCreated)274aba9d-124b-e070-0cde-953998035151
I found the solution in the thread linked above, specifically the reply that started with "How did you go about recreating the Managed Metadata Service Application?" For some reason, pointing the service application to the existing database when recreating the service didn't work and I had to create it with a dummy database and then use PowerShell to Point it at the old database.
Hopefully this helps someone else!
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