adb devices was working fine last year, but after upgrades and new software installs,
adb devices
no longer recognizes USB attached Android devices.
Her
If none of the above works for you just as it didn't for me, just try using a different cable. The cable I was using was my friend's cable but it was only meant to charge over USB, not to pass data over USB. I don't know how to tell the difference between micro USB cables that do or don't support data over USB but it's worth a try! Good luck!