There is something running in the background on your device. Most likely improperly. It's hung up on some kind of command...
If you are/were restoring from an iCloud backup, it may be stuck on that. (Those restores can only be done on WiFi and can take 1 to 4 hours per GB of data coming down)
If no restoration is taking place, try this...
Settings/General/Reset/Reset All Settings
This will not erase any data, it will simply reset the temporary preference files the internal CPU uses and go back to factory defaults. So you'll see your background go back to the spacey looking one, and your wifi passwords will have to be put back in... Siri and Location Services will be off. All of this stuff is easily repopulated, and it will most likely fix your problem.
There are deeper issues that sometimes require a full OS restore and setup as new device.. but usually thats a last resort.
You can also make a genius bar appointment at an Apple Store where they can run some diagnostics to see exactly what the hangup is. It will also check some hardware, but 9 times out of 10, this is a software issue.