For various reasons, spacecraft are sometimes "put to sleep" for periods of time on long voyages. Many systems are shut down, and radio communications suspended. Tracking of spacecraft at very large distances usually requires the spacecraft to receive the weak signal, amplify it, and rebroadcast it over a narrow beam from a high gain antenna if possible, so I'm guessing that sleeping spacecraft at large distances can't be actively tracked, although their trajectories can be forward-calculated with high precision.
This answer started me thinking about the whole process, and I remembered the videos of the crowd at ESA going crazy when Rosetta woke up.
In the video below it looks like the Rosetta spacecraft was hibernating for about 2.5 years. Is this the longest time that a space craft has "gone to sleep" with no tracking, and then "woke up", on schedule and in the right place?
The reason I say "on schedule" is that I am interesting in how long mission planners have been comfortable loosing contact with a spacecraft in an intentional, scheduled manner. Was it really without communication for 2.5 years? Was there also no tracking for the whole time?