Hello.
1) Its possible because of some error happens at your server.
Please recheck your error.log in your server configuration, about any relative errors. If you can not find error.log file, please contact support of your hosting company about helping in finding this file. After we will know about the reason of this issue we can fix it.
Also you can try to deactivate one by one all your active plugins at the WordPress > Plugins menu and make testing after each plugin deactivation. If its not help, please deactivate all your plugins and active the default WordPress theme. And then retest it again. May be there is some conflict.
2) Please note, Booking Calendar is use standard WordPress wp_mail function for sending emails. This function can be overridden by other plugins.
Please check this troubleshooting instruction https://wpbookingcalendar.com/faq/no-emails/
The most probably you need to check the point #5 from this instruction and install WP Mail SMTP plugin which is ‘sending emails via php’ option and it can resolve that issue.
Kind Regards.