Did you just lose the confirmation SMS/email you received?

Not an issue! BookMyShow has a way to retrieve it. 

You can now visit our website in.bookmyshow.com and click on the Resend Booking Confirmation option present on the bottom of the page.

Enter the same contact details as you entered while transacting, hit on "Resend" and there you go. The confirmation SMS/mail is with you again. 

You can resend it via mobile application too, there will be an option under Purchase History (if logged in at the time of booking).

It's that simple.