iPhone 4 is dead and won't charge.

I'm having a strange issue trying to charge my iPhone. I knew I had low battery and that it would die soon, and when it died I plugged it in. After 20 minutes, it still did not come on. I tried 2 different chargers and nothing. I've tried the computer and wall charger with all 3 cords. Left it plugged in for at least 15 minutes each time. I get the charging symbol on the screen every time, but it won't actually charge.
Anybody know what is causing this? I don't think Apple would even help if I took it in because I cracked the screen a couple months ago, unrelated problem but I'm pretty sure that voids my warranty having the cracked screen. Any help is greatly appreciated. Thanks.