Inability to Delete Text by Highlighting It

Microsoft Office Word

I don't know what I did, but I can no longer delete text in the following 
way: highlight the text and then start typing. Ostensibly the text that you 
start typing will delete the highlighted text. Or, I also can't highlight 
text and then delete it by pressing the "backspace" button. It's really 
annoying. Can anyone help??!
Tools | Options | Edit: "Typing replaces selection."