This works for me. Find (zombie?) server (can happen when quitting terminal with server running):
$ ps ax | grep rails
If it returns something like:
33467 s002 S+ 0:00.00 grep rails
33240 s003 S+ 0:15.05 /Users/Arta/.rbenv/versions/1.9.2-p290/bin/ruby script/rails s -p 3000
kill it, and run anew:
$ kill -9 33240
$ rails s