Whoops! You will definitely need your Core Pages in order for the plugin to function properly. Good news is that they're easy to restore.
You can restore your core pages by going to General Settings > Manage Install > Repair Core Pages.
Now you should be able to go back to building your site, all pages intact. If, somehow, your core pages were not restored, please submit a ticket to customer support and we'll work with you to resolve the issue.