Sometimes if the update application is abruptly stopped, then the lock remains stuck.
Then running
UPDATE DATABASECHANGELOGLOCK SET LOCKED=0, LOCKGRANTED=null, LOCKEDBY=null where ID=1;
against the database helps.
You may also need to replace LOCKED=0 with LOCKED=FALSE.
Or you can simply drop the DATABASECHANGELOGLOCK table, it will be recreated.