This may help if the touch screen digitizer is not damaged or already been replaced but still it doesn't respond. Try to check out the solution below and check the highlighted components, it is the basic parts where to check or replaced.
Nokia C7 Touch Screen Problem SolutionView larger image
Check the touch screen panel assembly for possible damaged, cracked or broken. Check the touch screen flex cable, there is an IC mounted on it. managed to rework it.
If the touch screen still doesn't work, check the pin connector, clean if oxidized. Check or replace the resistors coils and capacitors.