How did it “suddenly die”? Do you get anything at all? Have you reset the PRAM? I would also double check all the connections, specifically the LCD cable and make sure it didn’t pull loose from the BACK OF THE LCD. I would measure the voltage to the connectors as well, just to make sure the connectors are actually getting power. 2010 iMacs are notorious for GPUs failing as well, just an FYI
Well without looking it up, phones generally open one of two ways; The back comes off somehow (Screws, snaps, or adhesive) and you can get at the internals through there, or you have to pull the LCD off which would be generally held down with adhesive, that you would need to heat and separate.