that has happened to me a few times, and each time, the forms (and other objects so afflicted) were FUBAR. Try making a copy of the form and see if you can open the copy. if yes, work with the copy. try creating a new database, and import the form into new database. if the form works in the new database, work with the new database and import all of the other objects.
each time this happened to me, I was unable to fix the form itself.