Certified GKH Gardening Expert
It looks like the one remaining leaf has some black spots on it, which could indicate a Cercospora leaf spot infection. This could have caused the loss of the other leaves. Other diseases like Cylindrocladium blight or Botryosphaeria canker could also cause these symptoms.
In any case, you should remove any fallen leaves from around the rhododendron to prevent disease spread. Please see these articles for control advice and symptoms: