Can't get rid of mysterious line of tiny boxes

[normancamp]

In Word 10 doc. I have a number of lines of tiny black boxes that have appeared when I would hit return to go to the next line. Eventually I figured out that if I immediately hit backspace I could get rid of that line of boxes, but before I figured that out there were several such lines in place, and I can't get rid of those. Can't hit backspace and then return because the line just skips down the page a ways. Can’t highlight and delete. It’s as if the line isn’t there—but it is. ???