When I boot the MacBook Pro, it chimes and shows the Apple logo and progress bar. The progress bar gets roughly halfway, and the screen goes totally gray. There is no mouse cursor, and waiting 10 minutes or more shows nothing new. When I try to boot in verbose or verbose/safe mode, it does much the same thing: lots of boot message scroll by, and then the screen turns gray. Trying to do an Internet recovery also does much the same thing. Almost immediately after the download process ends, the screens goes gray.
The only boot method that works is Internet Apple Hardware Test. Both the short and extended versions of the test run to completion and show "No trouble found".
I have not yet tried to boot in Firewire Transfer mode. I will try that tonight.
I have reset NVRAM with command-option-P-R, and I did hear the second chime when doing so. This made no noticeable difference (except turning my boot sound back on, which had been turned off).
Where should I go from here?