hello aspirant,

you have mentioned all the details but you did not mention the exam/entrance exam you are referring to.

you don't have to worry about getting your registration number back.

now, since we do not know which exam you are referring to, so we just help you out.

stay calm on the initial basis.

visit the official website and then there will an option/link available which says forgot registration number,

after that the site will ask you some of the details and will send you your reg.number on the registered email id or the phone number.

hope that helps