My Samsung Galaxy S4 was working fine, all normal and then suddenly, it turned itself off. It refuses to turn on again. Doesn't respond to holding the power key. Plugging it into charger has no effect. If I take the battery out and put it back in again, it vibrates and the screen shows the Samsung Galaxy S4 logo (as it does when you usually start it up), but then the screen just goes black again and nothing happens. It's on an O2 contract and they do any necessary repairs for free, but I'd rather see if I can get it working myself before having to go in to the O2 shop and sending it away for a week. If anyone can help, I would most appreciate it