HP probook 450 G3 only shows solid white LED on the power button no fan spin no display just that

[Scramy09]

Laptop came to me as a dead laptop I just changed charger and it charged but when I switched it on I only had fan spin and every LED blinking .I tried USB bios update but did not work .One lucky day it switched on and i had display . When I tried to assemble it I just got the solid white light on power button nothing else. Any ideas on how to fix would be much appreciated. 

[WAWood]

@Scramy09 

We're not HP Tech Support, so we don't have any repair instructions or tutorials that we can provide.

To determine the cause with hardware failures, you have to be able run diagnostics. We have no way of accessing your PC from here, so we can not do that for you. You have to do it yourself. You do this by pressing the Esc key repeatedly when rebooting and then, when the HP Startup Menu appears, selecting Diagnostics (usually F2) and letting it run.

But if it is NOT possible to run diagnostics, then there is NOTHING more you can do by yourself -- and there is NOTHING we can do.