You accidentally committed your secret key to version control, or the server got compromised, or you lost control of the secret key for some reason. Using an online service to generate one is a bad idea - nobody else should ever have your secret key. How do you generate a new secret key? The otherwise-excellent Django docs are silent on this. Here's how Django generates one when you run startproject:
>>> import random >>> ''.join(random.SystemRandom().choice('abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)') for i in range(50))
Generate a new one and you're good to go.