We've got your back! BookMyShow has a way to retrieve it.
You can now visit our website in.bookmyshow.com and click on the 'Resend Booking Confirmation' option which is present on the bottom of the home page.
Kindly enter the same contact details which were entered while transacting and hit on "Resend" and there you go. The confirmation SMS/WhatsApp/Email is with you again.
You can resend it via mobile application too, there will be an option under 'Your Order' (if logged in at the time of booking).