Why is my iPhone 4 stuck in a boot loop?

I

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Hi,

I hope you can help me, my iphone 4 problem. I was just about to send a text when my phone off and i can't get to come back on and seem dead but when i plug it in a power supply the apple logo comes on for about 15 - 20second then off for 5 second and back for 20 seconds and that all it does all day.

I have taken out the sim card and put back in but with no luck.

Help me please.