My was at a criticaly low state with the white heart. upon plugging it in it read goodbye and turned off. It has done nothing ever since. No charge it doesnt even vibrate upon plugging it in.
Have you tried a different charger? Maybe plug socket blew a fuse. Failing that the USB connector on the phone may be faulty. Worse scenario...software/hardware fault. How old is the phone?
It might be something wrong with the connector, try the charging plate at any Microsoft store to see if that works and if it does, then buy one in an at&t store for 5 bucks.