I believe the reCaptcha service may be experiencing issues. I have multiple websites that are all receiving 404 errors. I know for sure at least 2 of them were working a couple of hours ago.
This is the error I am seeing: Failed to load resource: the server responded with a status of 404 (Not Found) http://www.google.com/recaptcha/api/js/recapcha_1-XXXXXXXX.js
I've also just found a few other posts with people experiencing the same issue.