Dear Shobha, there are many students who face such problems. Don't worry about this. One thing you can do is to ask your school incharge - who handles all such problems - to provide you the certificate. Just tell him the problem you are facing. Generally school provides certificates to such students as soon as possible. Other thing is, you can talk to the college authorities. Yes, sounds weird, but it's actually not. Many a times students fails to provide any required document, in these cases their seat allotment is cancelled generally. But if you request to College authorities and provide them with a affidavit - it totally depends on them whether they agree for it or not - mentioning the reason for not providing the certificate and the date by which you can submit that, they may not cancel your seat allotment.
All the best
Hope this helps