2 possible problems
1/ A blockage in the pipe , possible if you run off a well or there was some work done in the plumbing. This would mean taking turning off the water coming down from the attic and removing the ball cock in the toilet cistern and then checking if there is any blockage there. Then turn on the water slowly to clear out any debris making sure to have something to stop the water going everywhere.
2/ An airlock. Only way is to connect a hose that has mains pressure to the water outlet in the toilet cistern and try and clear the airlock. Also can get messy but it possible the easiest way to go. Just run the water for 10 to 20 seconds and then remove the host , this should cause the water to flow and will help to clear the airlock.