I was travelling from Kentish Town to Borehamwood. There was no announcement at Kentish Town, no notice explaining what happened. The (underground) staff at the station knew nothing about what was going on.
The departure board showed a train 2 hours ago as "delayed" with no arrival time, and a train terminating at West Hampstead being over 20 minutes late.
After waiting about 20 minutes, I gave up waiting and took the underground to Edgware and then take bus 107/292 to Borehamwood. Fortunately I can do this - I would be left high and dry if I lived in Radlett or St. Albans.
When I arrived at Elstreee & Borehamwood station at about 22:30, the departure board showed the next train as "delayed" with no projected arrival time - I assume all the trains before had been cancelled?