Tefal Anti cal steam generator resevoir blocked?

We have a Tefal steam generator anti cal, it is emptied on a regular basis, now the water from the resevoir will not empty into the generator tank.
The tank just pressurises and lets out a loud screeching noise, you relieve the pressure by pressing the steam button.
You can use it by constantly filling the tank but once the water has run out the process needs to be repeated, any Ideas?