I had that quite a few times now. Can't say if it has something to do with moving out of action-point range. But I doesnt automaticly mean, that you're completely stuck.
The last time this happened, the enemy had its turn but I was able to select and move my units. One of them got stuck like you described and the other moved just fine. After a short while, the enemy turn ended without any more actions taken by them and it was my turn. I could continue like usual.
