A white blank screen or freezing problem is sometimes cause by an interrupted firmware. You may need to restore or update the device firmware first before do a hardware troubleshooting.
Then if the update or flashing procedures doesn't work, you may then check or try to replace the LCD screen module. If the LCD is already been replace and the display problem still exists, you may then proceed to check the display filter chip on the PCB board. Refer to this solution below to locate it where it is being mounted.You can also use this solution for deeper tracing of the display connection line paths from the LCD pin connector down to the display filter chips, mostly when the device suffered wet or water damaged.
Nokia 6720c display connection line paths
There are two EMI-ESD filter IC's being used in the display circuit of Nokia 6720 classic. You may then try to replace each one these alternately to easily determine which one causes the problem.
In some factors of display problem such as garbled or saturated screen display only one filter IC is faulty and causes the display problem.