while typing an emal and I attempt to insert some text in the middle of a sentence that is already typed, it goes into overtype and deletes the words and types over them

I was typing an email and went back to a sentence that I had already typed toward the beginning of the email .  I then inserted a comment in the middle of a sentence, and while inserting the comment the text that was already there disappeared. The new text overrode it and deleted the text already typed.

It was in that overtype mode or whatever it's called where words cannot be inserted without removing the text that is there.  I didn't deliberately set it in this mode and don't know how to restore it to the insert text mode.  HELP!