How do I unlock my Samsung S5 SM-G900V. Just mysteriously locked up somehow.

[AC Question]

I used to connect my phone to my PC using the USB cable that came with the phone and be able to transfer files from phone to computer and vice versa. Now when I connect the two, the PC recognizes the phone is connected, but I click to access phone or SD card folders, but the folder is empty. How do I unlock my phone to restore access and where would I enter a password, if needed? Thanks for any help with this problem.