It looks like your theme, bigcity, includes an exact copy of one of Jetpack's Contact Form module.
To get back into your site, you'll need to log in to your site via FTP, and rename the wp-content/themes/bigcity folder to wp-content/themes/bigcity_old. Once you've done so, you'll be able to get into your site again, and your site will have automatically switched to using one of the default WordPress themes, like Twenty Fifteen, that do not include that code.
Another alternative would be to remove the wp-content/plugins/jetpack folder, if you wish to keep your theme.
If you're not sure how to do this, you can ask your hosting company for help. They should also have documentation explaining how to edit or remove files from your site using FTP or a File Manager in your admin panel.
Once you've made those changes, I would recommend that you get in touch with the theme author to report the problem.
I hope this helps.