Hello and welcome to justanswer, My name is ***** ***** i will be assisting you today,
I'm sorry about the issue, Please try to unlock your phone then plug it to your computer, if the problem persists please download Samsung Kies an Connect your android phone to your computer , via usb cable.
Samsung Kies will then open automatically. If not, manually open it by double clicking the Samsung Kies icon on your desktop. You can now transfer photos and files from your computer to your phone and vice versa.
Let me know the results
Has everything worked out, doyou still need help please?