This morning the Circle and Hammersmith & City Lines are each totally suspended. In addition there is no service on the District Line between High Street Kensington and Edgware Road.
This blog is not constantly updated, so please visit the tfl website for up to date information.
The Real Time Map shows the huge impact of these suspensions.
It is apparantly all down to 'Health & Safety'. The BBC website states the issue as being around the "Traction Brake Controller". I don't work on the railway, so may be wrong, but I think this is (or is connected to) the "Deadman's Handle".
It looks as though I may be on the Central Line today!