Hello there,
You have not mentioned which examination you are enquiring about. Nevertheless, don't worry, here's a solution. 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.
Once you recover your username, you can easily change your password by using the "forgot password" option. An OTP will be sent to your email id or contact number, and using that you can reset your password.
Hope this answer helps you.
Thank you!!