Worcester 28si II problem?

My boilers warning light comes on and the pressure bar goes below zero and nothing works, I have tried filling the system back up with water and restarting it like it states in the owners manual but nothing, I'm not sure if the flame as gone out so have tried to re light the flame and re start the system but the warning light just comes back on. from the out let pipe it sounds like air escaping so not sure whats up with the system.
Can anyone help

Pherring
January 2011