I was having a hard time getting my Rails webserver to restart after it having a bit of a hissy fit. The process to handle most apps when this happens is to locate the PID (Process ID) of the thing that is broken and kill it.
An easy way to do this with Rails is to type the following into your Terminal;
lsof -wni tcp:3000
Which looks for all processes running on port 3000 (the most common port for things like Webrick and Thin to run on) and returns them all as a table.
One of the columns of the table will be the PID for the process, copy it and paste it into the following line which will go into your Terminal;
kill -9 PID
Replacing 'PID' with the PID you just copied.
This will kill all processes relating to that PID, once this is done you should be good to go ahead and restart your rails server.