Dell Inspiron 5520 Screen back light not working

If you've tried replacing the screen multiple times to no effect, AND you've changed the inverter if it's a CCFL backlit screen (it's more likely LED), that would leave the backlight power supply to the LED as the cause -- for which you'll need to replace the mainboard.

It looks like there is a schematic for your board -- start there.

https://www.laboneinside.com/dell-inspiron-5520-7520-vostro-3560-schematic-diagram/

Mainboard is the main system board, aka motherboard, yes.  

@ejn63 

Thanks for your response and too could you please help me to identify the faulty parts on the mainboard. appreciate it if you could share some pictures or documents for my reference. I just would like to try soldering on faulty parts before throughout. 

And could you please confirm, are you referencing motherboard or some other parts in machine called mainboard 

@ejn63 Actually, I am facing these issues from when I removed the display cable from the screen and inserted again without turning off the laptop even power supply is also connected at that time. 

for these cases, is the issue with the mainboard? if yes.. is it possible to fix the issues without replacing the motherboard?

It sounds very likely the power circuit to the backlight is blown out.  Unless you have a lot of time on your hands and access to a hot air soldering station, any professional repair will exceed the cost of a replacement board (they seem to run between $150-200 depending on the type of video) by at least a factor of two.

Even if you do have the equipment and expertise, identifying and sourcing the faulty part(s) may prove a futile exercise.