The train was suspended between Eighth Avenue in Manhattan and Broadway Junction in Brooklyn, according to an MTA service announcement sent out at 7:37 a.m.
Service resumed about 8:14 a.m., but the MTA told riders to expect "extensive delays."
The MTA could not immediately be reached for comment.
Straphangers packed into the DeKalb Avenue station Friday morning vented about the snarled subway.
"What can I say? Improve it," said, Madeline Rivera, 59, who was on her way to a cardiologist appointment and was "definitely" worried about being late. "This happens all the time."