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:
./manage.py runserver
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.