Yesterday Ia drupal developer upgraded from Drupal 6 to Drupal 7. It was straight forward with one gotcha. I should have deleted all files and folders apart from sites. I did not ....
So I switched theme to Garland, turned off all modules and copied the latest Drupal head over the my Drupal 6 files and ran update.php.
However what I failed to anticipate was that a left over block.php.tpl in Garland from Drupal 6 was causing my Drupal 7 theme to lock up.
The workaround was to copy "modules/block/block.tpl.php" into "themes/garland/", replacing the old D6 block.