The Apple iPhone relies on the use of a touch screen that lets you access features as well as answer incoming calls. If your iPhone is ringing but not displaying the screen, you may find that you are unable to answer calls when they come in. There are four possible causes for an iPhone that constantly rings with no display. By analyzing each cause of an iPhone with display failure, you can usually fix this issue.
The iPhone has a proximity sensor that automatically deactivates the iPhone's screen during calls in order to conserve the battery. Occasionally, this sensor may cause the iPhone to ring but not display anything on the screen. This typically occurs when a finger or something else is placed over the proximity sensor, which is located directly above the touch screen area of the iPhone. If this is causing your constant ringing problem, you can resolve the issue by making sure that nothing is touching the sensor when you answer phone calls.
An iPhone that constantly rings but has a blank screen may be caused by an accessibility bug. This problem commonly occurs with iPhone 3G and 3GS phones that run iOS 4.0 or lower, but may also occur with the first generation iPhone. The bug occurs when the Zoom accessibility option is turned on. If you suspect that this is the cause of your constant ringing problem, you can disable Zoom by tapping "Settings," then "General," then "Accessibility," and then the "Off" button next to "Zoom." Alternatively, you may find that upgrading your iPhone to a newer version of the iPhone firmware fixes the problem.
The iPhone may lock up due to having too many open applications or an unstable application that is currently running. When the iPhone is locked up, you may find that phone rings yet you are unable to answer it because the screen is blank. In this case, the problem can typically be fixed by restarting the iPhone. You can perform a regular reset by holding down the "Sleep/Wake," button until "Slide to power off" appears, then use the slider to turn the phone off; if necessary, you can perform a hard reset where power is temporarily cut by holding down the "Sleep/Wake," and "Home," buttons until the slider appears, and then use it to turn the iPhone off. To turn the phone back on, hold the Sleep/Wake button until the Apple appears. The phone will restart.
If the proximity sensor and accessibility settings are not to blame and restarting does not help to resolve the display problem, the iPhone may be defective. It is possible that either the LCD connector for the iPhone is disconnected or the LCD screen itself is bad, which may cause the iPhone to ring with no display. This may occur after the iPhone has been dropped or may even be caused by a manufacturer's error. You can get your iPhone repaired or replaced by contacting Apple or taking the iPhone to an Apple Store Genius Bar.