Some of the responses speak of clearing cache, etc., but that is something I do regularly, and NotePad always opened with the desired HTML code. Suddenly, NotePad opened with nothing in the page.
For your problem: Make sure that "notepad.exe" is present in your PC. I did a search for all occurances of "notepad" and came up with two 65K versions in the c:\windows and c:\program files\internet explorer\connection wizard. There was a third file in c:\windows\system32, but it was only 3K. I opened it with an editor and found an "MZ" file identifier (which means nothing to me). But, renaming this file to "Xnotepad.exe" made the HTML code suddenly reappear when I click on View Source. This anomaly just seemed to happen, and I have no idea how it got there.
I hope this helps.