Hello there,
We cannot recover your registration number, you have to do it yourself. Don't worry, we are here to guide you. Please check the inbox of your registered email id. The authorities might have sent you the necessary credentials during the verification process. Check the spam folders too. You should also search your messages of your registered mobile number. If you don't find there either, you can send an email writing about your problem to the authorities conducting the examination. You can find the helpline number and email id in the conducting body's official website.
Hope this answer helps you.
Thank you!!