Tablet [Q] Nexus 7 stuck in fastboot mode and cannot be recoganized

I tried to install android L today and did the following things:
1. adb reboot boot-loader
2. fastboot oem unlock
3.'m in linux)

The third step stuck in 'sending system'. I googled and was encouraged to change a cable. So I unplugged the device and killed However, when I plug it to pc again, it can't be recoganized. I cannot boot into OS either

Please Help.