"Communications with the STEREO Behind spacecraft were interrupted on October 1, 2014 immediately after a planned reset of the spacecraft performed as part of a test of solar conjunction operations. There have been no successful communications since then, though attempts to recover the spacecraft continue. Here we explain the events that led up to the loss in communications, and the activities that have been carried out in an attempt to recover the spacecraft. As explained below, initial indications are that a series of problems in the guidance and control system led to the anomaly."
STEREO Behind Spacecraft Experiencing Communication Problems, Earlier story