It can happen when you update a plugin. Something in the update accidentally breaks something else.
Alan suggested https://www.wpicalavailability.com/ which looks identical to the plugin those of us who did his old course use, called WP Simple Booking Calendar. You’ll need to buy it, install it, activate it then configure a calendar in it with all your bookings. Then you need to copy the shortcode for your new calendar and finally, on the right page, replace the current one with it. Bob’s your uncle.