Dead iPhone 4. What to do?

Yesterday I found my iPhone 4 shut off and obviously I thought it ran out of battery. However, as I tried to charge it didn't charge ( I tried two chargers), and now it is completely dead. I can't connect to iTunes, I've tried to hard reset (lock+home), can't enter DFU. It's just a plain brick, which is odd because it was in my pocket.

Any ideas what it may be? What should I do?

Thanks