Usb Problem

[Android Question]

So when I plug my phone up to my laptop via usb, there used to be a thing that popped up on my phone that gave me the option to only let it charge, or plug it up like a camera and access my files on the computer. I can no longer find that and the option doesn't come up in my Ongoing in my notifications. Anyone know how I can fix that? Its a galaxy s3 also.