When emails stop getting generated, it could be because cron is running. Check the status of the cron by going to Reports --> Status Reports
If the Cron is running, wait for few seconds to a minute till the Cron completes. Then log out, login back, clear all cache, go to the content, edit it and save. This should solve the issue. If it doesn't, clear cache again, edit the content, change the Send Email to field to someone else's email, save it and then edit it again and change the value back to original.
If none of the above solves the issue, rebuilt the rules cache, clear the cache, edit the content and save.
As a last resort, use a different browser/different computer to login, clear cache, edit and save content. Once solved, go back to the original cmputer/browser.