Water tank not full: If you are using the machine in water tank mode, you may just be out of water. Refill the tank and see if the PID turns on.
If you are using the machine in plumbed in mode: Verify that the switch in the back of the machine is set to "RETE" and not "TANICA".
The water tank magnet is not being sensed: Verify that the magnet in the bottom of the water tank is not floating away from the wall of the water tank, which can cause the machine to not be able to sense it. You ideally want the magnet float against the wall of the tank. This can be accomplished with something long and slim, like a zip tie or a knife.
The water tank is sitting too far away from the wall of the machine: Similarly to the magnet floating too far away from the sensor, the actual water tank itself may be sitting too far from the inner wall of the machine. Again, this will cause the machine to not be able to register the magnet inside the tank.
If none of these are the case, you can try to put a powerful magnet on the 2 screws located where the magnet sensor is to see if that will turn the PID on. If this works for you and the PID turns on with a powerful magnet, you may just need a new water tank float. Contact us at firstname.lastname@example.org if this is the case.