Hello Madhav,

You have not mentioned any test name so I'm assuming you are taking about JEE - Main. If you have lost your hard copy of your roll number you check the e-mail account you gave during the enrollment for the test or you might get an SMS on the mobile number on the phone number you used for your test registration. Not only for JEE - Main but most of the exams conducted in India send the roll number on your E-Mail ID as well as your phone number after the registration process.

All The Best.