iosiphone
Since the upgrade to iOS 6, I've been unable to access my mail on wi-fi, cellular or disconnected.
The symptoms are:
This is a hardware problem. This comes up when the ribbon connector from the camera to the main circuit board comes loose or gets disconnected altogether. If you have the right tools and enough DIY skills to open up your iPhone, it's a very simple fix that should be fairly obvious to see. Otherwise, the Apple Store should be able to repair it for you (I would think free of charge, but who knows).
If you do want to take apart your iPhone and attempt the repair yourself, PowerBook Medic has some really nice instructions for taking apart all versions of iPhones from 3GS and above. They also sell replacement parts and will repair it for you as well if you want. (Disclaimer: I have no affiliation nor prior experience with this company, so use at your own risk.)
This is what the front camera of an iPhone 4 looks like:
That little white/silver part is what's probably come loose; just pushing down on it gently should allow it to snap right back in.
Seems like a solution is to hold home button + volume down + screen lock until device forcibly reboots. After that, guided access began responding properly again and I was able to exit the app.
Edit: Through trial and error, I felt this was home button + volume down + screen lock, but as comments have noted, volume down is not necessary.
Best Answer
First, force-quit Mail.app. To do this: Go to your home screen. Double-click the home screen. Locate Mail.app in the bottom bar that appears. Press and hold Mail.app until the icons wiggle. Delete Mail.app from the bottom bar by clicking red X.
Try again to launch Mail. If this doesn't work, reset the iPhone by pressing and holding power and home buttons for 10 seconds. Restart device once it shuts off.