ComBoxes Are Inactive

I added a combo box to each sheet of a workbook. Each box referenced a range within the sheet and linked to a cell on the sheet. Each box was tested and worked perfectly. Yesterday. On opening the book today, however, I've discovered each combo box is dead in the water. They are totally inactive and can't be activated in any way. I've checked the properties sheet for each and can't see anything out sorts. As a test, I added a new combo box to a sheet, and interestingly enough, it not only works fine, but also seems to have activated the original box, which also now works fine. Other boxes on the other sheets are still inactive, however.