Greetings aspirant!

No need to worry if you have lost your admit card. If you are a private student you can easily re-download it from the official cbse website, after answering few questions and filling in your details. If you are a regular student you will have to contact your school regarding it. In most cases school keeps a copy of admit cards. If not your school authorities will definitely help you in this.

Hope you find it helpful.
Thankyou!