Random start up (chime but black screen)

Hello,

At first, excuse my English writing: I'm French but I find more solutions on English forums.

There is a very strange and random thing on start up on a friend's Macbook Pro 15" Early 2011 that I try to repair.

At first, I have to say that the discrete GPU is out of order : solved by software way (https://apple.stackexchange.com/question...). For sure, the problem does not come from it.

A few weeks ago, my friend gave me back the Macbook : black screen and no start up apparently but always the chime plays and the battery led is on.

I suspect the Macbook of having fallen : the body presents obvious bumps (back cover and keyboard cover).

I tried many, many things without understanding when the Macbook Pro starts up really.

Sometimes, it's just a question of time: waiting 1 or 2 hours and it starts up alone.

Sometimes, I have to press firmly on the body or on the keyboard (for example near the space bar or near the top of the body or somewhere else on the keyboard) and it starts up immediately.

Sometimes, nothing happens but after a period of time (3-4 minutes) and pressing firmly again on the body it starts up.

It would seem that the body heat and pressing firmly the body play certain role. Not sure.

Don't know if my explanations are very clear...

But if you have any suggestion...

Many thanks in advance.