Ubuntu doesn't recognize Android devices anymore

Ubuntu 14.04 doesn't recognize any of my Android devices anymore. (Tried with the Nexus 7 and Galaxy Nexus)

I thought this was an MTP or ADB-problem, but connecting / disconnecting the devices doesn't even change the output of lsusb or dmesg at all.

It's like the devices aren't even plugged-in. However they both charge normally, when I plug them in, so I can't imagine having a cable-problem or a problem with the android devices themselves.

(I've tried it with usb-debugging on and off on both devices - no difference)

I have problems with my Ubuntu GNOME 14.04 installation recognizing my Nexus 7 at times, too. This is what works for me:

1. Make sure the cable can transfer information, that it's not just a power cable.
2. Make sure you have USB debugging enabled.
3. Unlock your device.
4. Connect your device to your computer.
5. Your device should display a pop-up, asking if you want to accept a connection to your computer. (If you had this pop-up before, you might have ticked the "always accept a connection from this device" checkbox.)
6. Normally, your device should now pop up in your favorite file manager. If it doesn't, check if ADB finds your device. In a terminal, type adb devices (you might need to be root).
7. If ADB lists your device, go to Settings > Storage and select the options button in the upper right corner:

8. Select the file sharing option you want:

The problem was ultimately the cable.

Some USB cables are just for charging. Other cables also support data. I tried 3-4 different cables, until one of them ultimately worked.

That was the reason, why connecting the phone didn't even register with dmesg at all.

if dmesg doesn't report any change, it's definitively a cable problem or in the worst case a problem with the microUSB port of the Android devices