I've had the Nexus 5 for a few months, and exchange files between my PC and the phone using the USB cable frequently. I also charge the phone using the USB connected to my PC. Usually, the window pops up asking what I want to do with the device, which I ignore if I'm charging, or select Explore if I'm file-transferring. About a week or so ago, I noticed that instead of options, there were NO options on the pop-up, and just a "Do Selected every time" or whatever. But no options below. Until last night, I haven't had the need to transfer files, so I ignored it.
So last night, I closed the popup and looked in My Computer, and the normal Nexus 5 was nonexistent as a Drive. There was a Safely remove Hardware icon, so I selected it and chose Open Devices and Printers. There was my Nexus 5, under Devices, identified as Media Player under Properties. My only other options on right-click are Eject, Create Shortcut, and Troubleshoot.
I can no longer access my files. I rebooted both, checked the settings on the phone, called T-Mobile, then Google, then Iolo, my antivirus and whatever-else-it-does doer, to no avail. The kicker is, I plugged it into a buddy's computer hours earlier to retrieve a file, and it connected as storage immediately. I tried on my wife's laptop, and it does the same. So something had to have changed inadvertently on my PC, and I'm sure it's something simple, but it's beyond me.