What did you try to change the admin location?
If you renamed any folders, files or modified the htaccess file or some thing other than modify local.xml that is possibly why the site has fallen over.
If you tried to change it in system > config this in theory should work but in my experience it usually just causes the admin to not work. To fix this you need to open phpmyadmin, go in to the config and remove the entry for this.
The correct (and best because it always works?) way is to edit the app/etc/local.xml file as Moshe indicated.
Near the bottom of the file there is a line like “<frontName><![CDATA[admin]]></frontName>”. Change “admin” to what ever you want the admin location to be, clear the cache and that should be it.
Hope this helps!