MacBook Pro (late 2013) w/ Yosemite & FileVault won't boot: LCD light only, no other activity
The Apple Store Genius quickly resolved the issue by doing an SMC reset  and NVRAM reset in rapid succession. 

(He may have done more than one of each; I'm not sure. I don't think he waited for an audible startup chime before adding the NVRAM step. It was a 5-minute visit and the store was loud.)

So if this state recurs, I'll try each of those in order, in more combinations/timings to be sure. 

The Genius indicated that he'd seen the same issue with another person earlier that same day, and that the FileVault full-disk-encryption was an involved factor: the black-but-backlight-on screen may have been showing in lieu of, or perhaps 'in front of', the usual pre-boot/decrypt-volume prompt screen.