Lost drop down menu

I'm an Excel rookie so help please. I successfully created some lists and used them for data validation, showing the allowed list in a drop down menu in the relevant cell. All of a sudden, the drop down indicator has disappeared. I've checked the validation by simply typing in words from the list and not from the list and it works fine. When I go back into data validation, the box for a drop down menu is checked so it should be there. I've tried taking off all validation, tried deleting my lists and starting again, everything - but the drop down indicator in the cell is still missing. Can anyone tell me where to find it????????