You ran the deprecated command service nginx status and got output similar to this:
 error@vmtest-ubuntu1604:~$ sudo service nginx status * nginx.service - A high performance web server and a reverse proxy server Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: en Active: active (running) since Mon 2019-03-04 10:44:30 EST; 13s ago Main PID: 16843 (nginx) CGroup: /system.slice/nginx.service |-16843 nginx: master process /usr/sbin/nginx -g daemon on; master_pr |-16844 nginx: worker process `-16845 nginx: worker process Mar 04 10:44:30 vmtest-ubuntu1604 systemd[1]: Starting A high performance web se Mar 04 10:44:30 vmtest-ubuntu1604 systemd[1]: Started A high performance web ser lines 1-11/11 (END) 
 Upon which you didn't get the terminal prompt back.
 First, this command is deprecated because Ubuntu 16.04 replaced upstart with systemd. The service command now attempts to translate your command into the corresponding systemd command, in this case systemctl status nginx. This is what was actually run, and what actually generated the output you saw. In a future Ubuntu release, the service command will be removed entirely.
 What's going on?
 By default in current versions of systemd, systemctl pipes the status output through a pager, by default less. 
 You can turn this behavior off by passing --no-pager in the command, in which case the output will just be dumped to standard out, and your terminal comes back immediately.
 error@vmtest-ubuntu1604:~$ sudo systemctl --no-pager status nginx * nginx.service - A high performance web server and a reverse proxy server Loaded: loaded (/lib/systemd/system/nginx.service; enabled; vendor preset: enabled) Active: active (running) since Mon 2019-03-04 10:44:30 EST; 3min 33s ago Main PID: 16843 (nginx) CGroup: /system.slice/nginx.service |-16843 nginx: master process /usr/sbin/nginx -g daemon on; master... |-16844 nginx: worker process `-16845 nginx: worker process Mar 04 10:44:30 vmtest-ubuntu1604 systemd[1]: Starting A high performance web... Mar 04 10:44:30 vmtest-ubuntu1604 systemd[1]: Started A high performance web ... Hint: Some lines were ellipsized, use -l to show in full. error@vmtest-ubuntu1604:~$ 
 
 Note that this output isn't meant to be machine-parsed. If you are trying to check service status in shell scripts. you should use other systemctl commands, such as systemctl is-active.
 error@vmtest-ubuntu1604:~$ sudo systemctl is-active --quiet nginx && echo Running || echo Stopped Running error@vmtest-ubuntu1604:~$ sudo systemctl stop nginx error@vmtest-ubuntu1604:~$ sudo systemctl is-active --quiet nginx && echo Running || echo Stopped Stopped