Hii!
Firstly it would be better if you specify the name of the exam, you are enquiring about.
Secondly, while registering for the exam, you must have got an email or a text message in which your registration number and password must have been mentioned. Search for that. Probably you will get the registration number.
Else, you can contact the respective helpdesk of the  related examination and ask them to sought the problem out.
They may help you.
Hope this helps.
Good luck!