Have an iPhone 3G and was working perfectly up until last night. Went to bed and plugged it in as normal, phone had died about 3 hrs previously. Left it to charge overnight and tried switching it on this morning but it would only go as far as the apple logo and then power down again. There is a good charge in the battery but it just won't go past the apple logo before powering down by itself. Any ideas as to what could be causing this?