did you uninstall plex from rockstor page first?
I guess you didn’t 
What you can try is the following from ssh:
docker pull linuxserver/plex
This will redownload the plex image. After that run this:
docker ps
It will show running containers. like this for my box:# docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
30c3dc18b951 linuxserver/sabnzbd “/init” 21 hours ago Up 21 hours 0.0.0.0:9090->9090/tcp, 0.0.0.0:8085->8080/tcp sabnzb
59d10124acb4 4737cca51096 “/init” 3 days ago Up 22 hours plex-linuxserver.io
9247cb8df092 6cee52827bdd “/init” 3 days ago Up 28 minutes 0.0.0.0:11000->8181/tcp, 0.0.0.0:11000->8181/udp plexpy-linuxserver.io
46d6ccdf81cb a165d6f35451 “/init” 3 days ago Up 21 minutes 0.0.0.0:8989->8989/tcp Sonarr
fb17c062d08a 353dc75755bb “/init” 3 days ago Up 15 minutes 0.0.0.0:7878->7878/tcp radarr
If you see plex running there do this:
docket stop plex-linuxserver.io
After that try to start it with:
docker start plex-linuxserver.io