For whatever reason be it jailbreaking or fixing an error dfu mode is used by most advanced iPhone users
1. Power off the device.
2. Hold down the power button for 8 seconds
3. While holding the power button hold the home button for an additional 7 seconds
4. After the 7 seconds release the power button but keep holding the home button
5. After about 7 more seonds you can release the home button
You should see a black screen if you do then you probably did it right
If you are unsure then you can do this with your device plugged Into your computer and judge whether or not the device is really on by the sounds the computer makes.
To come out of DFU mode I usually just hold down the power and home button together for 8 seconds to force my device to reboot