You probably added some code that Word doesn't know how to read, so it gave
up on showing the doc in WYSIWYG mode (What You See Is What You Get).
The answer is: don't make a website in Word. Word is okay for webpages
that you create entirely in Word, turn into HTML, and never touch again.
But it is not designed to make websites. If you want to be editing and
refining your pages, you are going to need more control over the HTML than
Word allows you.
You might try Nvu, which is free, and has a WSYIWYG and a code view that you
can switch between, similar to opening the doc in Word and then in Notepad,
but much easier. Word doesn't have a code view, so there is no command to
switch out of it, which is what you are asking for.
http://www.nvu.com/
Word also makes bad webpages, with a lot of excess code--all that excess
code is designed to ensure that Word will be able to open the doc as a Word
document--but it makes editing the code in Notepad much more complicated and
difficult for you.