[support] Disappearing user login

[Roger]

We have a most peculiar situation.
While developing a theme in zen starterkit and following tuts and 
videos, etc the user login ceased to function.
Cannot log in even with localhost/mydrupal/user which is how I normally 
login. All we get is a blank screen.
Google suggested some overriding code to go into the page.tpl.php, which 
allowed login but returns the new theme screen but no admin even if we 
enter localhost/mydrupal/admin in the url.
/user is core functionality, file template and css are all available, we 
have not changed any core files.

I used drush to make garland the default theme and tried to drush 
disable the zen theme but both fail. drush returns an error request 
-drush clear-cache drush which in turn returns the same error.

I did drush up to install drupal 7.25 and update modules but it did not 
help.
Cannot find anything we have done in the dev theme which may relate to 
/user login or admin.

most perplexing.
How would we solve the lack of admin and why would login cease to function?

Help much appreciated
Roger