Plug in the phone using the wall socket charger, leave it for a few hours to get a charge (i know it doesn't look like its charging). Later, press the on/off button. The battery icon appears with just a thin red line. Let it go back to black. Then press the the on/off button and the menu button simultaneously for about 8 seconds. A red button should appear on the screen with a question asking if you want to power off the phone. Say yes. (I think it was a slide bar, but can't remember now.) Wait a few secs while the phone powers off. Then remove the power plug from the phone. Now reinsert the power plug. After a few (worrying) seconds of nothing, a white Apple logo appears in the middle of the screen, a few more seconds and the iphone main menu appears. No data has been lost, no settings are altered, and best of all, the battery shows a full charge and seems to be back to normal again.