Well, obviously the head has to come off, but you need to find out why that valve failed to close. It could have been hit or seized as others have already mentioned.

Myself, I'm wondering if you used new or used valve springs - a weak spring could have let the valve remain open a tad too long and *bang*.

If you pop the spring off, you can tell a lot. If you can move the valve easily until the last, it's probably just bent. If it's stiff, you have too little clearance, and it was seizing. This will help steer you clear from these troubles next time.

Let us know what you find.

Mark