Hmm… not a great day for my iPhone last week. Its wi-fi was playing up so I decided to power cycle it. After switching on it hung on the Apple logo for 4-5 minutes. Power cycled again – same problem. Sounds like a common issue. Here are some helpful suggestions for fixing it. Anyway I was lucky – I connected the iPhone to the Mac (with iTunes running) and, after a couple more restarts, the iPhone booted successfully.
I say “successfully” in that it got to the Home screen. But then every third-party app I tried to launch quit immediately back to the Home screen! (Apple apps were OK.) Once again, this sounds like a common problem. Ouch. In my case I managed to fix it by connecting the iPhone to the Mac, running iTunes, right-clicking the iPhone in the left-hand pane and choosing Restore from Backup. It took ages and hung right near the end (I had to yank the USB cable out of the phone to stop it), but after that all apps launched no problem.
If you’re reading this because you have this problem, and the above solution doesn’t work for you, you may need to do a full system restore (as described in the linked discussion thread above).
Nasty. Hope it doesn’t happen again…