If you have a query that shows it is being blocked by session ID -2, this means you have an orphaned distributed transaction.  You might see this surface in Activity Monitor or in To kill the orphaned transaction you will first need to get the transaction GUID.  You can find the owner GUID of the transaction with the following query:
USE master;
SELECT DISTINCT request_owner_guid AS UoW_Guid
FROM sys.dm_tran_locks
WHERE request_session_id = -2;

After getting the GUID you can kill the transaction with the following KILL command. Replace the GUID with the request_owner_guid returned from the previous query:

KILL '<replace with guid>';