Hello everyone

I recently granted a fully funded PhD course in Material department of Birmingham University.I applied for the ATAS but yesterday found it to be rejected. I'm really upset about it and it really changed my life plans!!can anyone help me in this case?
My nationality is Iranian. anyone know why do they reject students in ATAS?They don't give you any reason for it.however I have the right to appeal but wanted to make sure about how to do it.

Many Thanks
Arash