When connecting my Nexus 5 to my Windows PC, it was NOT showing up in Windows Explorer. This also happened with my Nexus 7.
First things first – make sure you have the proper USB driver installed for your Nexus. Here is a link to download the Google USB driver. Another solution is to use a Universal ADB (Android Debug Bridge) Driver for Windows, developed by Koushik Dutta in April 2013. Just click here to download and install the driver – it will work for most all reasonably popular Android devices.
Once you have the proper USB driver installed, you may still have problems browsing your device from Windows Explorer. Here’s what I needed to do to fix the problem:
Go to Device Manager. At first, the Nexus device showed up under a heading of “Android Device”.
Uninstall the driver and delete it (check the box).
Next, unplug and replug the device. It will then show up under Portable Devices.
Uninstall this driver too. Then, unplug and replug again. This time, Windows will install the correct driver, and you should be able to access your device’s data files from Windows Explorer.