Hi Amreen Kaur,
Thank you for reaching Microsoft Community. We're sorry for the trouble this has cause you.
Will you confirm which Surface device do you have, was it the Surface Pro 4 or the Surface Laptop 4?
The system update might not properly install or got corrupted. If your device stuck on a BitLocker screen, refer to this link on Finding your BitLocker recovery key in Windows.
Important: If you are unable to locate the BitLocker recovery key and can't revert any configuration change that might have caused it to be required, you’ll need to reset your device using one of the Windows recovery options. Resetting your device will remove all of your files.
Kind regards,
Lavenia