When I use my back end url to login to joomla for editing, it will not load--how can I login?
Think of what has changed since it last worked..
Did you update Joomla? Did your host upgrade the php version on the server? This may be the case and if you go in and delete an ampersand at line 106 you might be all good.. but in case not;
Edit your configuration.php , set error_reporting to maximum. Refresh admin login page and see if it gives a better clue. Make sure to set it back to default when you're done testing.
Frankly, if it's not due to a php version issue, I'd probably just download a fresh copy of the exact Joomla version you're using, unzip on your local machine, and ftp the full cms package up to your sites web root (overrighting existing files). This usually get's things going again.