Dear aspirant,

You have not mentioned the exam of which you have lost the admit card . Every exam have their own process of retrieving the roll number or admit card. In some cases the official website of a conducting body of a particular type of exam , allow the candidates to write their birth date or registration number and it will automatically generated their roll no.

So you need to contact the particular authorities through phone or email and ask them what you should need to do so that you can get your admit card or roll number.