If the javascript in your header tags is your problem as you suspect, you have a couple of options:

USING the DNN UI (web browser):
If you don't have direct access to your DB you'll have to use the DNN UI:

1. Disable JavaScript in your browser. 
   In IE7:  TOOLS->INTERNET OPTIONS->SECURITY->CUSTOM LEVEL->ACTIVE SCRIPTING (Disable)
    
2. Now, on your website, click your "HOME" menu link.  This will change your URL to show the TABID.  Using the URL as it now appears, change the end of the URL from this:
    
      /Default.aspx
    
   to this:
    
      /ctl/Login/Default.aspx
    
3. You should now be on your login page without the header javascript active.  Log in as normal with ADMIN or HOST account.  (If you still can't log in then javascript is not your problem.)
    
4. Now you need to get to your SETTINGS page.  In the URL, chage the end from this:
    
      /Default.aspx
    
   to this:
    
      /ctl/Tab/action/edit/Default.aspx
    
5. Remove the JavaScript from the page header tag setting and click "Update".
    
6. Re-enable JavaScript on your browser.

 
USING the DATABASE:
If you have direct access to your DNN database tables then you can do what you need in one step:  Go into table "TABS", find the entry with TabName = "Home" and clear out the "PageHeadText" value to remove the page header tags.

Good luck-
-mamlin

I created the same problem (and re-created it in test) by selecting an alternate page as my site "Login Page", but did NOT have the login module loaded in a container in a pane on that page, or on any page viewable by 'all users'.  Dumb, but it happened.

Luckily I had a second portal that I could log into as HOST, then once logged in I could change back to that portal, and fixed it.

Admittedly it was my bone-head error, but a warning in DNN would be nice...

I did not try directly changing the database since I wasn't sure what to do.  Changing the Default.asp at the end of the URL did not work, but that may just work for the java script issue...

For info, I would like to find out what table/field exactly in the database needed to be changed.  I'll play when I have the time...