What has happened to my iPhone 4S' Battery?

I sent my iPhone 4S in to have the wifi repaired. When it came back everything worked A-okay initially. Then during a second charge, I would plug it into a charger and then nothing for a VERY long time- it wouldn't turn on or show any signs of life. Then I came back later and the iPhone showed a 100% charge. Once I removed it from the cord, the iPhone died at once. Later I put it in and waited for an eternity for the 100% charge to show up. The top kept saying searching... I wanted to try removing the AT&T SIM but by the time I came back the phone was dead again (I hadn't taken it out of the cord) I've tried USB and my PC isn't recognizing it (yes, it usually does.) Right now I'm waiting again for the phone to light up while it's in the charger. WHAT IS GOING ON?! Thanks! :)

UPDATE: The phone keeps showing the Apple logo trying to turn on while charging but never comes to the slide to unlock screen..