There are two more things to try. If the first thing you try fails, try the next.
Two Button Reset.
This is the least likely to result in loss of data. Read that again, especially the phrase "least likley."
- Unplug the phone form the computer.
- Press and hold both the home and the sleep/wake buttons (ignore the red slider) until you see the silver Apple logo.
- When you see the logo, let both buttons go.
If this worked, you should see an icon instructing you to connect to iTunes. My guess is that the reset did not work.
This will very likley result in loss of data.
- Plug the iPhone into the computer.
- Ensure that it is running the lates version of iTunes and that it has access to the internet.
- Press and hold both the home and sleep/wake buttons for exactly eight seconds.
- Release only the sleep/wake button but continue to hold the home button for another five seconds.
- Let go of the home button, then look at the computer screen for an instructional popup.
The popup should say "iTunes has detected a device in covery mode. . .(etc)" Click accept to all boxes and let iTunes download and install all of the latest device Firmware and Software.
If no popup appears, repeat the above DFU instructions. If you 've never done a DFU, it may take about three attempts to get the timing right.