If you are using Form Assembly to manage your Events forms and the site goes down; no one will be able to register for your Events. By following these steps you can configure a basic form using TargetX Events forms and quickly switch all your forms to use TargetX forms so that you can continue to use Events.
This typically happens in Orgs where Communities are enabled and your total number of users passes 1000. You'll need to go to your Event Custom Settings and click the box for 'Restrict Event Ownership':