It will not open, or you can’t select the text? If you can’t open the page source and your using Firefox press Ctrl+U, If you have Firebug installed then press F12. If you’re using IE (then get Firefox, chrome etc) or press Alt, View –> Source. If it still fails to show you the source code maybe you have set an alternative HTML editor in your internet options and it’s not opening/corrupted.
If you can open the source, then the browser will show the source but only viewable view and you cannot edit it, for this I would recommend Firebug with Firefox :)
Good article here about Understanding Source Code