It would be best if you'd check with the cinema's Lost & Found first, alternatively you may drop an email on helpdesk@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.
Once you email us the details, we will give it our best shot to help you find it.