Swathi, As you haven't given the details of the exam for which you have forgotten your user id or password, therefore, I am answering this query in a generic way.
So, If you have forgotten your user id and password used  for logging in and want to retrieve it, then you should click on the "Forgot Password" link which is available on the webpage where you are needed to enter your user id. By clicking on the link, you would be redirected to a page where you are asked to enter your email id or mobile number or any other details which was asked during the registration period. Complete the procedure as directed on the "Forgot Password" page and your password would be reset or new password and user id would be probably sent to your email id.

If still the problem persists, then you should contact the Helpdesk of the examination through email or mobile and the details required for the same would be available on its' official website and they would be better equipped to solve your query.