Why won't my iPhone connect back onto the Sprint network?

Does the phone show its IMEI under settings? If not, you have damaged the baseband which can happen accidentally when prying the battery up.

Take the motherboard out and see if you can see any missing components or damaged/loose ones around the edges if the motherboard.

If the phone just says searching and is on iOS 6, there is a known issue after any repair when you disconnect the battery that you need to manually reset the date/time--but this is less likely for you since you we're connected to wifi and it would have set automatically.

That would be a board-level repair which would mean you'd need to be experienced and equipped to solder surface mounted components under the microscope. Procedurally, you'd just locate the damaged components on the logic board, then replace them with the same components taken off another board.

This really is a professional-level repair. The good news is that the phone still has some resale value for parts.