Have you been re-ordering or re-creating fields in the catalogue?

As per the documentation, the default fields must not be edited.

Yes, I think you must have changed fields around. The grid of images display is the default, but if you change the position of fields it will break the references to them.

This is what the eCommerce tutorial says:
You may add additional fields, such as extra product images or specifications. However do [b]not[/b] re-order the original fields (or delete, or change the field type), as the system uses the order of these fields internally to know what they refer to. If you wish to change how they are shown do it via template editing.