Recently troubleshot and repaired a iPhone 4S. I will share this information with you, perhaps it will help you get your iPhone up and running again saving you lots of $$$. At times the digitizer screen will have to be replaced if the following solutions do not work for you since it might truly have failed.
There are a few things that can cause an iPhone 4S to display a black screen. If the sound is still working you can rule out the possibility of a dead battery. This iPhone 4S I will working on had the black screen but sound could be hear and also it would ring when its number was called, everything was functional just no video on the screen.
Most causes can usually be solved by restarting, resetting or restoring the iPhone to its factory settings (the last resort). If none of these solutions work for you it most likely is a hardware problem, i.e screen needs swapping out or the connector is loose and needs to be re-inserted. They can become dislodged from impacts or drops. The nice point about these phones is that removing the 2 bottom screws near the charging connector lets you slide the back off the phone giving you access to the internal electronics..in the upper top left part of the phone under a small metal plate are many of the connector plugs that you can check.
Apple suggests a restart as a first troubleshooting step. A restart is done by pressing and holding the “Sleep/Wake” button for five seconds. Normally, a red slider appears on the screen, but you probably won’t see it since your screen is black!. Place your finger a half inch from the top of the screen on the left side and then drag your finger to the right edge. The slider doesn’t require much precision in where you place your finger so, assuming the touch controls are still working, this should work. After a half-minute or so, turn the iPhone back on by pressing and holding the “Sleep/Wake” button.
If restarting your iPhone doesn’t solve the dark-screen problem, reset the device. Do this by holding down the “Sleep/Wake” button and the “Home” button at the same time for at least 10 seconds. Hopefully, you will see the Apple logo appear before you release the buttons. If you don’t, release the buttons anyway and then wait a minute or so for the iPhone to power down and turn back on again. If resetting the iPhone doesn’t work the first time, try it once again.
Restore to Factory Settings
Restoring an iPhone to its factory settings is generally the last hope you have to resolve a black-screen problem yourself. To do this, connect the iPhone to your computer and launch iTunes. After selecting the iPhone in the right corner of the screen or in the left sidebar, if you activated the sidebar option, click the “Summary” tab and then click the “Restore iPhone” button. When the restore process is finished, the iPhone should display a gray screen with the word “iPhone” on it. Drag the slider at the bottom of the screen and follow the prompts to set up your iPhone again. This process restores your iPhone to its original factory condition. During the process, you are prompted to restore a previous backup to return your files and data to the phone.
Recurring Black Screen
After your iPhone is working properly again, update to the latest iOS and update your apps. For example, some iPhone 4S owners reported black-screen problems after upgrading to iOS 6.0, but these issues were resolved with the release of iOS 6.1. If the iPhone goes black again after updating and applying a backup, you may need to repeat the restore process, this time without the backup. Bugs between an app and the iOS may also be the cause of a black screen.
Hope this helps you, Good Luck!