Siemens 6DR4004-8NN30 malfunction

Hello everyone,
Need some help. We have an ARCA make valve which is controlled by Siemens SIPART positioner. There is a NCN position sensor, Model 6DR4004-8NN30, connected with the valve which gives feedback to the positioner. Suddenly the link connecting the position sensor to the valve broke and valve started hunting from 0-100%. Upon checking, we found that the roller of NCN position sensor is not moving at all. It freezed at that point. Reason for this is not clear as NCN sensors are very rugged.
Any sort of help is much appreciated