okay, then it's time to get your hands dirty and start looking for clues
Firstly I would check the server error logs (I assume you've been through those 4 links above 1st)
You can get to those if you have CPANEL installed, via here:
There might be errors getting logged with XF... but as you can't access the admin control panel, it's obvisouly not all that straight forward to check... but all is not lost. These errors are logged in your database, so access your database (with phpmyadmin or what ever you use), the find the table "xf_error_log" Is there anything recently listed there?
This was weird i couldnt login to my control panel with my iphone4s but it worked to login with my friends account on the iphone.
Now im on my computer and it workes again to login with my account it have never been a problem to loggin to the panel with my iphone before what happend?