what happens to my online form when it reaches its limit

If you are on the Free Plan and you go over the given 100 Submissions Limit then your forms would become disabled until you either waited for the free plan to reset on the first of each month or by upgrading your account plan to Bronze or higher depending on your needs.  An error message stating that your form is "over quota" will be displayed to users filling out the form.

Please note that once you get close to your limit a notice would be sent out to you ahead of time to serve as a warning that you are about to hit your plan limitation. You can compare the different plans and their limits per each plan level here: Http://www.jotform.com/pricing