So the phone shut itself off?
You didn't pull the battery because it froze from trying to delete the messages?
I know that when you delete a lot of messages, it will freeze for a few. And you have to use the Home key to get out of it.
Either way, it sounds like it's beyond being fixed by yourself.
The only thing I know to try is to Boot into the boot loader.
Hold the "Down" part of the Volume rocker, and the power button at the same time. If they phone boots into a white screen with Androids riding skateboards you are in the Bootloader screen.
If you can boot into that screen, it means the phone should not be bricked. But you may have to go to Sprint to get it repaired.
If it doesn't have a response to holding that down, then it may be bricked.
Either way it seems like you need to head to your local Sprint Repair center. And make sure they know it happened today so even if you bought it at launch you will still be within the 30 days.
Good luck to you, and Happy 4th of July! (Even though it's now the 5th)