I've never had this problem before.  Usually I just rebuild the assembly and it'll work fine, but now I can't place lids on ANY openings whatsoever.  I can open any part in the assembly and place lids on it, but in the assembly itself I cannot.  At first I tried selecting the planar surface around the opening and nothing would happen, but now when I click on the 'lid' icon it just depressed it and the interface does not even open.