try temporarily setting site Error Reporting to Maximum to see if there are any meaningful error messages
try temporarily enabling debug
if using ChronoForms v4, try setting Relative URL to No in the form General tab
if using CAPTCHA with cache enabled, try with cache disabled
if using CAPTCHA, try temporarily disabling CAPTCHA or try RECAPTCHA instead
Reference: http://chronoengine.com/forums/viewtopic.php?f=2&t=65518&p=311707&hilit=form+not+submitting#p264431