Hi CHuguley,
It is always helpful when posting run-time problems to show the line of code where the problem occurs. You can find this out by clicking the Debug button on the error message dialog box.
From what information you have provided my guess is that the problem is occurring on a line of code that looks like:
Sheets("some sheet").Visible = True
and that the problem is occurring because no such sheet exists with that name. I would recommend that you look very carefully at the name of the sheet in the VBE project pane, paying particular attention to blanks. Even leading and trailing blanks in the sheet name must be matched exactly by the text strin used in the Sheets() or Worksheets() references. If you don't see the sheet in the VBE project pane then you have accidentally deleted the sheet and that is why the code can't find it.