tkbharani, most likely, your crontab file has been truncated/removed. You can check that by using 'ls -l /var/spool/cron/crontabs/your_user_name'. This will show whether your file exists and is zero bytes or doesn't exist at all (though I think that it would give you a different message in that case).

Regarding restoration of the file, you will have to talk to the administrators. They should be able to restore this from any OS backup.