Perhaps you have a field that has disappeared from view off the end of the canvas - at the top right of the form editor, there is a dropdown list containing all the fields on your form. Try selecting each field in turn in that list, and check the value of the 'y' coordinate on the field properties. If it is a very high number, overtype it with a small number so that it comes back into the visible area of the screen - you can then either reposition or delete it as required.
