Turbine has announced via the Lord of the Rings Online Facebook page that the Helm's Deep expansion launch is being delayed until Wednesday, November 20th. LOTRO servers have had a rough few days with a power outage at the main data center. According to news published on Facebook, the backup servers did not kick in resulting in the need to restore data, hence the delay of the expansion launch.
You can keep informed as to the progress of the server restoration by heading to the Lord of the Rings Online Facebook page.