Hiii

Although you have not mentioned the name of the exam you are asking about but i suppose you are asking for AIBE.
Do not worry as a lot of students are facing the same problem. It is possible that there is some serve issue which will be resolved soon. All you can do is  make sure that your application is correctly filled and is accepted. And also keep trying to download the admit card. 
Even if the problem persists then please put up your query through mail to the authorities and ask them to solve your issue.

Thanks!