if you didn't change ANYTHING, it's possible one of the libraries you are including added another JS library like scriptaculous that is messing up your $ reference.
- [jQuery] Firebug gives a "$ is not defined" error elvis_wu
- [jQuery] Re: Firebug gives a "$ is not defined" er... MorningZ
- [jQuery] Re: Firebug gives a "$ is not defined"... elubin
