I have a MacBook Pro late 2011 and have had the following repairs/issues (over past couple of years):
Replaced (by an agent - 2016) faulty graphic card (under warranty) as per forum https://www.geekzone.co.nz/forums.asp?forumid=47&topicid=190761
I replaced the HDD (in 2017) with https://goo.gl/QMDFBH (500GB Samsung 850 EVO SSD)

Now I have an issue where the MBP won’t boot up - the progress bar gets to about half way then the screen goes grey!
I’ve attempted to: reset SMC, NVRAM, safe mode - I get to the spinning world, choose wifi, gets to progress bar and then grey screen.
Performed a (extended) hardware test with a “no trouble found” result.

My guess is a service agent but thought I’d attempt this forum first!

Appreciate any advice.

Thanks
Maurice