Android ADB device offline, can't issue commands

I can't connect to my device anymore using ADB through the command line or in Eclipse.

Running the command

adb devices

returns the device name, but it says it's offline.

Things I've tried.

1. Toggled Android debugging mode
2. Reinstalled the Google USB driver
3. Restored the OS to a previously working backup (CyanogenMod)
4. Swapped the USB cord
5. Rebooted the phone/computer multiple times
6. Updated the Android SDK

I really don't have any clue what's going on. Anything else you think I can try, I'm all ears.

To be clear, if you're having this same issue the problem is probably an out-of-date SDK. As of 4.2.2 there is a security feature that requires you to confirm the RSA fingerprint of the connecting device. Open the SDK manager and update the tools! Then reboot.

Try running adb devices after running adb kill-server. Security question pops up after that. Worked for me.

I just got the same problem today after my Nexus 7 and Galaxy Nexus were updated to Android 4.2.2.

The thing that fixed it for me was to upgrade the SDK platform-tools to r16.0.1. For me, this version was not displayed in my SDK Manager, so I pulled it down from http://dl.google.com/android/repository/platform-tools_r16.0.1-windows.zip directly.

You then need to rename the platform-tools directory and unzip it to android-sdk-windows/platform-tools. Using the SDK Manager, I had also updated to the latest sdk-tools before this.

If your whole Eclipse and ADT are ancient, you may need to update them as well, but I didn't need to.

Note: you may need to run SDK Manager twice (once to update itself) before you will see the latest packages.

It also seems to occur frequently when you connect to the device using the Wi-Fi mode (in Android Studio or in the console by running adb tcpip 5555 for example).

To fix:

1. Disconnect the USB connection—or turn off the device's Wi-Fi if you're connected over Wi-Fi.
2. Close Android Studio/Eclipse/other IDE.
3. Run adb kill-server to ensure adb is not running.
4. Restart your Android device.
5. After your device restarts, connect it via USB and run adb devices. This should start the ADB daemon. Your device should now be online again.