Hello Brenda, If it is happening from more than one computer, it is likely a server side issue. You will want to have the designer enable the error reporting on WordPress to see the specific error. I am guessing this is a memory issue, though the error reporting should tell you exactly.
The Customizer can sometimes demand more memory, so you will want to first try and increase the allowed php memory in the php.ini file for the account. Your designer should have no problem changing the setting for you.