Hi dear,

You have not mentioned the name of the exam you are appearing for and for which you are unable to download your admit card. Hence it is difficult for me to tell you the exact source from where you can get your registration details back. However, most of the exam, the authority sends an email and text message on the registered number while registering. I'll recommend you to check your email, and messages for your registration details. Also, apart from this you can contact the helpline of exam conducting body to get your details.

Good Luck