Why won't my iPhone 3GS turn on?

I've literally tried everything, but it won't turn on.
It randomly just shut off without any warning.
I had it on sleep mode, and it didn't even show the turning off screen or anything, and now it's stuck on a black screen. I've tried everything, resetting, trying to turn it on and off, pressing the home button multiple times, charging it, it just won't work. I don't understand! It was only half-way into its life span and now this happens!

Try holding the power button and the home key for a long time when switching on

If i dosent work try selling it at buybackzone.com they pay allot for broken phones its crazy!

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