VIM enters multi-line editing mode; how to exit?

I start writing a new document. Very shortly, VIM enters a multiline editing mode which I do not intend. All of the lines of the paragraph I am editing get underlined and I cannot navigate any particular line in the block, or make any edits. CTRL-W makes the underlining go away but it returns with the next keystroke. HELP! 🙂 Thanks in advance. ..alex…