How would you diagnose and resolve a blocking problem where many queries are waiting on a single long running transaction?

You would query dynamic management views like sys.dm_exec_requests and sys.dm_tran_locks to identify the specific session holding the blocking locks, examine what that session is currently doing, and then either wait for it to complete, optimize the underlying query causing the delay, or in an emergency, safely terminate the blocking session if appropriate.

SELECT blocking_session_id, session_id, wait_type, wait_time
FROM sys.dm_exec_requests
WHERE blocking_session_id <> 0;