I am working my way through Tutorial 2 and in mid session I find that I can no longer go into design mode. When enter http://localhost:3000/silverstripe/admin (which has always worked previously) it now comes up with the Log in screen but there is no input box to allow the entering of the username and password. The part I was working on in Tutorial 2 was the automatic adding of the comment facility to every ArticlePage.
I was having the same problem (I think), the html was there but both the Email and OpenID forms were hidden.
I put up with it for a while and just used Firebug to display the form I wanted, then I deleted the OpenID module and that seemed to fix it.
Thanks for all your help. My apologies for the delay in replying. Although none of your suggestions directly appeared to make a difference the problem suddenly disappeared and the system went back to working normally. The only event that happened in this period was that there was a power outage that took down my PC.