Yesterday, I plugged in my Nexus 5 to do some inspecting for mobile web development like I do on almost a daily basis. When I went to "chrome://inspect", there was an exclamation point saying I needed to update chrome and wouldn't allow me to inspect any devices until I did so (It had been working fine just a few days before).
So, I let it upgrade to chrome on my desktop to 39.0.2171.65. After the update, I went to chrome://inspect, and lo and behold, I get the dreaded "No devices detected...". So, I go through a whole process of unplugging the phone from USB, revoking USB authorizations, completely reinstalling chrome on my desktop, reinstalling the USB driver, trying two different cables etc. Still, "No devices detected...". Yesterday, my phone just upgraded to Lollipop and still "No devices detected...".
Today, I restored my phone to factory settings. Still "No Devices detected..." I tried Chrome Canary. Same results.
I don't know what else to do. I even tried it on a different laptop using Windows 7 (using windows 8.1 on the other machine). I don't know why this would all of the sudden stop working.
This issue is resolved on stable-channel (39.0.2171.95) and beta-channel (version 40.0.2214.28).