docker出现Cannot connect to the Docker daemon.

This appears when using the docker command. At first, I found it very strange and there is no solution. The point is that this problem also appeared at a critical moment. Since I just got in touch, I don't know what is going on.
root@server rancher1.6.5]# docker load
Cannot connect to the Docker daemon. Is the docker daemon running on this host? Cannot connect to the Docker daemon
. Is the docker daemon running on this host?
The solution for this. Just start docker again!
[root@server rancher1.6.5]# systemctl enable docker
[root@server rancher1.6.5]# systemctl restart docker