Docker commands do not respond anymore

I just had a similar issue as well. Rebooting the server did not work for me. I got this issue, because I just installed a new container with some kind of errors. After that, most Docker commands did not respond. I fixed it by executing the following command:

docker system prune -a

This removes all unused containers. In my case also the container I just added. More information:

https://docs.docker.com/engine/reference/commandline/system_prune/

That Docker commands hanging bug happened after I deleted a container.

The daemon dockerd was in an abnormal state: it couldn't be started (sudo service docker start) after having been stopped (service docker stop).

# sudo service docker start
Redirecting to /bin/systemctl start docker.service
Job for docker.service failed because the control process exited with error code. See "systemctl status docker.service" and "journalctl -xe" for details.

# journalctl -xe
kernel: device-mapper: ioctl: unable to remove open device docker-253:0-19468577-d6f74dd67f106d6bfa483df4ee534dd9545dc8ca
...
systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
systemd[1]: Failed to start Docker Application Container Engine.
systemd[1]: Unit docker.service entered failed state.
systemd[1]: docker.service failed.
polkitd[896]: Unregistered Authentication Agent for unix-process:22551:34177094 (system bus name :1.290, object path /org
ESCESC
kernel: dev_remove: 41 callbacks suppressed
kernel: device-mapper: ioctl: unable to remove open device docker-253:0-19468577-fc63401af903e22d05a4518e02504527f0d7883f9d997d7d97fdfe72ba789863
...
dockerd[22566]: time="2016-11-28T10:18:09.840268573+01:00" level=fatal msg="Error starting daemon: timeout"
systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
systemd[1]: Failed to start Docker Application Container Engine.

Moreover, many zombie Docker processes could be observed using ps -eax | grep docker (presence of a "Z" in the "STAT" column), for example docker-proxies.

After rebooting the server and restarting Docker, the zombie processes disappeared and Docker commands were working again.