You didn't mentioned about which examination or website you are asking this question. Now in general if you don't receive the necessary OTP to get your username and password then I would recommend you to check some time later because due to server errors many times there are delays in OTP. Apart from that if you still don't recieve it then it can be a error of your email ID or your network service. So recheck the details or contact the organizers of the examination/any helpdesk if available.

I hope this answer helps. All the very best for your future endeavors!