The same happend to me, hold on, I would go and seek the save game (and play the game so to seek the planet were this hapend)
The planets are of the core worlds and their names are "Athisa" and "Pind-Ra-Vosk", they have pops that stop their development (there was another planet of the core worlds witch has a pop with 0.01 grown speed, so that would be stuck soon once the other pop, one that is growing faster, start to work)
PLUS: I have a fleet forever lost in obliviom (Lost in action), its a small fleet, but its lost :c