First thing that you should probably do is check your PHP error logs and let is know what your seeing for errors.
Next I imagine that you would probably want to recover/make a new admin password. Here is how to do that:
http://www.tmib.net/content/recover-reset-admin-account-passowrd-osticket(http://www.tmib.net/content/recover-reset-admin-account-passowrd-osticket)