Mid 2012 retina MBP display won't wake from sleep

This problem is occurring with greater frequency all the time: I rarely shut of my MBP... I just close it to enter sleep mode. When I reopen the display, all I get is a black screen. The only way I know that the machine is even running is that the keyboard backlight is on.

The only way I can get the display to work again is to do a hard shutdown by holding the power key. When the machine is restarted and the display once again works normally, it's clear that keyboard inputs were being recognized while the display was black.

I tried using an external monitor via HDMI and had exactly the same results.

I've reset the SMC but not Vram---doesn't appear to be a Vram related issue. (I'm happy to be proven wrong!)

This is both frustrating and disconcerting. Help?!