Moto G3 (6.0) - Can't access files, tried many guides
The Moto G 2015 has known issues with this, and there is a fix most of the time...

Disconnect the USB cable
Navigate to Settings - Apps
Press the Overflow Menu (3 dots in upper right), and select Show system
Find External Storage and Media Storage
Open each item and select Storage
In this screen for each app, press Clear Data
Reboot
After the system appears up and working, wait a minimum of 5 minutes after the phone is fully booted. Databases are being rebuilt for these applications, if you have large SD card with lots of media files (several thousand or more) this may take longer to complete but there is no way to monitor it and know when it's done.
Reconnect the USB cable
Pull down the Notification Panel
Select USB Connection and change it to MTP. Note there may NOT be an icon in the status bar.

MTP connections use the External Storage app, and by association the Media Storage app, for USB access to the device. There is no direct access or &quot;mass storage mode&quot; anymore, it is all software emulated.
If this fails, a full factory reset is usually required. In rare instances, this does not fix it most models have firmware available on XDA, but that step likely is beyond the scope of this question.
Of course, all this depends on a working USB port on the phone and a good cable... although it is extremely unlikely, it is possible for the USB port in the phone to be defective.