i tried to update my iphone to ios 7.2.1 and now my phone is frozen what do i do!?

Reset the iPhone.

Press and hold the Sleep/Wake button and the Home button together for at least ten seconds, until the Apple logo appears.

If you can unlock your iPhone now, tap Settings > General > About then scroll to: Version

If it still says the 7.1.1, then the update was not applied.

Use Recovery Mode. You can restore from a backup > iOS: Unable to update or restore