If I am not wrong, you would have tried to modify the controls structure.
(i.e. you would have added or deleted any control on user form). It may be first
time with you but quite often you experience this problem if once your form is
working perfectly and afterwards you add or modify controls.

To come away with this problem export your module to some place, save and
close VBA project, again open your project, import module and run it. I hope it
would work  perfectly.