Transport for London said the service between Hyde Park Corner and Arnos Grove had been suspended in both directions.
The disruption was also causing minor delays on the rest of the line.
The problem started shortly before 7am this morning. TfL said it was trying to fix the train but had no time for when the issue might be resolved.
It was advising commuters to seek alternative routes, including local buses and Great Northern Train services.
"Please take the first train to arrive and change where necessary to complete your journey," TfL said.