A spokesman for Northern Rail said: "Due to a fault with the signalling system at Meadowhall, all lines are blocked.
"Trains may be cancelled, delayed or revised."
Disruption is expected throughout the early afternoon while the problem is fixed with alternative transport options being made available.
Rail passengers can use their rail tickets to travel from Meadowhall Interchange on the Supertram into central Sheffield.
Lines running into Sheffield and towards Rotherham and Doncaster are both currently blocked.
Network Rail staff are on site trying to resolve the issue