Are you on the latest Android Version called 6.0 / Marshmallow? When you plug your phone to your computer via MicroUSB / USB 3 cable you need to drag down from the notification bar / status bar then select the tile called 'USB for charging, Touch for more options' then select 'File transfers' for the Use USB for window dialog.
This should then allow you to access the files on the PC.
f that still doesn't work then you will need to go to phone settings, scroll down to About Phone, then tap on that, then tap on build number 7 times and it will show the developer options where you need to go back to the settings list to select it. Once you get in developer options scroll down and look for Enable USB debugging. Enable that and see if that works.
Other wise you may need to go into storage under phone settings then press the menu button, usually the 3 dotted menu button on the top right of the screen and change the USB connection settings there.
Still having issues? Look at the comments right below my answer for another solution.