What causes a laptop not to display at all, I have changed the display chip and also ene chip but still it powers but no display, what's the solution please?

Here are some proven diagnosis, for video chip the laptop should work with external monitor (TFT /CRT) that shows it's video chip because the machine is ok but display has a problem.
if no difference check on memory reset them properly or change memory from working machine to be sure.
Check the hard disk also when its short circuit it can cause the laptop not boot at all.
If the problem persist service the whole laptop and test different components separately.
then the conclusion will its motherboard hence replace board or buy another machine depending with cost effective.

Step1.
Remove battery and remove power supply Step2.
Hold down power button without power supply for about1min Step3.
Plug in you power cable and push you power botton Screen should come on at this point Step4.
Put back your battery and continue If this doesn't work, maybe your screen is bad or your connectors is having problem

check the cable connected between laptop monitor and laptop mainboard

Step-1 Detach the VGA cable from the laptop mother board, and connect the laptop external VGA port to the external monitor.
Step-2 Check the CMOS battery voltage if it is low then there will be no display.
Step-3 If these things does not work then the problem is in the mother board.