I am having this problem with the backspace key

I was in the middle of typing a long document without any issues. All of a sudden, I can't use my backspace key to delete text. I tried to do as suggested: Go into Tools>Options>Edit and check Typing replaces selection. The box was already checked but I unchecked and rechecked it just in case. Did not work. I copied and pasted the document into a new Word document - did not work. I notice when I open a brand new document the same thing happens in randome places on the document. It has become so difficult to type and correct errors, it is driving me crazy. Any suggestions would be welcome - using Word 2010. Thanks!