Which phone do you have? Did you receive the Marshmallow OS update? If so, the USB default is now charging only. Once you connect the phone and computer with the USB cable, pull down the phone's notification shade and change the USB mode to File Transfer.
On the phone, did you change the USB connection type from charging only to File Transfer?
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When you pull down the notification shade, tap where it says USB for Charging and you'll get a pop-up showing that it's set to Charging. Change it to File Transfers.