http://news.bbc.co.uk/1/hi/england/london/8520999.stm

Some kind of signalling problem near Liverpool Street area nuked the
service at around 5.30pm. The above story says the whole line was out
of action - a later radio travel report I heard suggested that the
line wasn't running between White City and Leytonstone, maybe the
situation changed over time though. Regardless, most definitely not
the sort of thing that should be happening.