Which version of MS Access you are using on which you area facing this issue?
Also, did you created This sub form using Wizard.
Do you see the sub form listed under form object?
Do you see the empty sub form object appear empty on form or it don’t appear at all?
If you see the forms listed under form object, open the form to see if they work fine.
Post that, open the design view for form with which you are facing this issue. Right click on the object. If its not here than add one from toolbar.
right click on it and add it.
Try this steps and let me know.