This is most likely the result of scaling your web dynos down to 0 dynos. To fix it, scale your web dynos to 1 or more dynos:
$ heroku ps:scale web=1
Use the heroku ps command to determine the state of your web dynos.
2010-10-06T21:51:37-07:00 heroku[router]: at=error code=H14 desc="No web processes running" method=GET path="/" host=myapp.herokuapp.com fwd=17.17.17.17 dyno= connect= service= status=503 bytes=