Sounds like maybe the game got corrupted or encountered a bug, not really what this forum is for, see the sticky thread for submitting bugs.
You may want to try editng the save and removing the problem from there, if you do not know how to do that, then you may want to ask for some help in the Scenario and editing forum.
You may want to try editng the save and removing the problem from there, if you do not know how to do that, then you may want to ask for some help in the Scenario and editing forum.