New Iphone 3gs won't turn on or connect to itunes?

I've just bought a new iphone 3gs, It was locked to o2 so i called and had it unlocked. I then put my vodaphone sim into the phone. Now every time i try to turn it on or plug it into itunes it randomly switches off and comes up with the apple logo again! So frustrating! Any solutions?

When I got my phone unlocked I had to plug it into iTunes first to get it to work and have you get the latest version of iTunen, or press the middle button and the bottom one at the same time hold it down for. Couple of seconds to reset it. That's all I can think of

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