STM32F429-DISC1 White Screen Error
This behavior is normal. The LCD will not display an image unless the firmware running on the target device has configured it.

By loading "a LED toggle program", you have erased the demonstration firmware which was displaying a user interface on the LCD. If you no longer have a copy of this firmware, you can download it from ST's web site. (A precompiled hex file is in the Demonstration/Binary directory.)