docker not working

ubuntu@ip-172-31-55-180:~$ docker version
Client:
 Version:      1.12.1
 API version:  1.24
 Go version:   go1.6.3
 Git commit:   23cf638
 Built:        Thu Aug 18 05:22:43 2016
 OS/Arch:      linux/amd64
Cannot connect to the Docker daemon. Is the docker daemon running on this host?
ubuntu@ip-172-31-55-180:~$ docker-machine ssh default
docker-machine: command not found
ubuntu@ip-172-31-55-180:~$ docker run hello-world
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.
ubuntu@ip-172-31-55-180:~$ sudo service docker start
start: Job is already running: docker
ubuntu@ip-172-31-55-180:~$ docker run hello-world
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.
ubuntu@ip-172-31-55-180:~$ sudo service docker restart
docker stop/waiting
docker start/running, process 32119
ubuntu@ip-172-31-55-180:~$ docker run hello-world
docker: Cannot connect to the Docker daemon. Is the docker daemon running on this host?.
See 'docker run --help'.
ubuntu@ip-172-31-55-180:~$ docker ps
Cannot connect to the Docker daemon. Is the docker daemon running on this host?
ubuntu@ip-172-31-55-180:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.5 LTS
Release:	14.04
Codename:	trusty
ubuntu@ip-172-31-55-180:~$