Need not panic, first thing first, it would be best if you'd once check with the cinema's 'Lost & Found'. Alternatively, you may drop an email on lkhelpdesk@bookmyshow.com with the following details:
1) Booking ID
2) Your contact details(Contact No & Email Id)
3) Details and description of the article lost.
4) Location and time of the article is lost.
The moment the above details are with us, we will give our best shot to help you find it.