Thought these might help
The 4 steps to solving almost any non-hardware fault on an iPhone.
Turn Phone Off/On - Hold the power button until the "slide to power off" icon appears. Slide to power off. Leave it off for about 30secs - 1min. Turn it on again using the same button
Reboot - Press the home button and the power button at the same time. Ignore everything you see on the screen until the Apple logo appears.
Reset - Settings > General > Reset. Options for erasing all, network settings etc. Network settings one is handy for data speed/coverage issues. WARNING - this option will make your phone forget the wifi password for your home broadband router.
Restore - Connect your phone to your pc using the data cable. Open iTunes, wait for your phone to appear on "devices" (left-hand side, under "library"). Double click your phone's name, until you see a picture of the phone appear on the right-hand side. Click "restore" (a button underneath the phone's picture) to perform this function. iTunes should back up your content before it does this.
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What to do if your iPhone goes dead and stops responding, including turning on.
Plug it into the mains using the USB charger. Perform a reboot (see above) while plugged in. If Apple logo appears within 20 seconds, let go. Otherwise, keep holding. Soon "Connect to iTunes" screen should appear. Connect to iTunes for a forced-restore. If all else fails to return to O2/Apple for repair/ replacement (if under warranty).
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Dropped your phone in water?
DO NOT air dry, blow dry or shake the water out of the phone. This moves the water to other parts of your phone. Place it in a sandwich bag with dry rice. The rice inhales the water naturally, drying out your phone. This is the best chance of getting your wet phone back to a working condition.