Most likely, the upgrade is stuck because an existing query on the database is holding onto a lock that the migration needs.
To resolve, login to your lemur database and run:
SELECT * FROM pg_locks l INNER JOIN pg_stat_activity s ON (l.pid = s.pid) WHERE waiting AND NOT granted;
This will give you a list of queries that are currently waiting to be executed. From there attempt to idenity the PID of the query blocking the migration. Once found execute: