Greetings aspirant!

There are some ways you can try to get back your admit card.

• Try contacting your school, schools usually keep safe the data and records of previous students. So writing a letter addressing principal or personally going to enquire can help
• you can write to concerned board ( cbse , icse, etc.) Requesting a duplicate admit card.
• as far as 10th admit card is concerned it is required for age verification purpose you can find alternatives to get the information mentioned on your admit card.

If you have done your class 10th from cbse than you have an option to generate duplicate admit cards.  http://www.cbse.nic.in/sch_loc.htm

if you still can't have it, you can also contact cbse help desk. http://cbse.nic.in/newsite/contact.html

hope you find it helpful.
thankyou!