Either you have installed or modified a theme that just breaks everything, or you've deleted the current theme without disabling it first.
- Check you have installed the theme in /sites/all/themes
- Check if the Linux ownership of the theme files is correct
- Check if you have installed the necessary base theme if required by the theme
If you see some content but no navigation blocks
We can fix up the themes by going directly to the required pages.
- You'll have to be logged on, so enter http://example.com/?q=user in the browser. Logon as normal. (use your own Drupal path of course)
- Enter http://example.com/?q=admin/appearance (D7) or http://example.com/?q=admin/build/themes (D5/6) to see a version of the themes page. Select a valid theme and things should be in a state where you can continue.
If you see no content at all, maybe errors or a totally blank screen and you cannot login anymore.
Remove the files of the bad theme and clear the cache. After clearing the cache you will be able to login again.
The main difficulty is that you have to clear the cache without being logged in.
Try one of the methods for clearing the cache described in http://drupal.org/node/42055.
Method 2: Change the active theme with Drush or in the database
If you have drush, the command to type would be
drush vset theme_default garland
Either on the commandline, or via an administration interface (eg PHPMyAdmin) enter the following query
UPDATE system SET status=1 WHERE name = 'garland';
UPDATE variable SET value='s:7:"garland"' WHERE name = 'theme_default';
Note that 's:7' refers to the length of the following string. Modify as needed. This is database surgery, tricky stuff.
If you are using per-user themes, and you've just messed it up for yourself as admin, try
UPDATE users SET theme='garland' WHERE uid = '1';
Be careful, as getting either of those lines wrong can mess things up just as badly.