I've had this issue a few times; always with units on the water, I think always on 500 maps. Currently have it with 2 battleships northeast of Fiji in game 2,989,740. There used to be a way to find the unit, then give it commands; in effect, reactivating it. I believe it was Ctrl+F. However, when I have hundreds of units (including stacks), I don't feel like spending a half hour to search for it. For what it's worth, I prefer legacy, and zooming in and out does not work.