Error: device offline

Question

The emulator is running. But when I write adb shell in my shell I get:

error:device offline

What is the reason for

Answer 1

I came with this condition twice.The first time I used the command "adb kill-server" in my PC, and restarted the android device. It worked. However the second time the method didn't work any more.This time I disconnected and reconnected the network. It worked.