Here is the explanation regarding the above query.
If you'll kill the process, it will stop the wagtail server.
First search for your process id (PID), filtering by "manage.py runserver"
ps aux | grep "manage.py runserver"
You will see something like this:
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
user 38 0.0 0.0 0 0 ? S 2433 0:00 python manage.py runserver 0:8000
Then you can kill your process using kill -9 $PID, in this example $PID is 38:
# Terminate running process
kill -9 38
Now you should be able to start your app normaly with using:
Restarting your Linode server will also solve this, but its like killing a fly with a hammer.
I hope that above explanation will be helpful to you.