Did you just delete 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 or application and Click on Resend Booking Confirmation option present on the bottom of the page on the app and on the top right hand of the website. 

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

It's that simple.