TeamWin “No OS Installed” Android Nexus 5 I lost my phone in Peru and had to downgrade to my old Nexus 5. It was rooted so I couldn't update the OS. In trying to unroot it, I [somehow] deleted the OS, and now the phone won't boot. I get to the TeamWin recovery options and it will say "No OS Installed, are you sure you want to..." How can I fix this?

Note: nexus 5 does not allow for external SDs to transfer files, etc.
You need to reflash the stock Google Factory Firmware image for your device with fastboot. 

Instructions and images are available here. Make sure to use a quality USB cable... we have had several bricking issues flashing firmware on Nexus 5 devices because of defective or poor quality USB cables.