MacBook – Retina Mac shows gray screen when waking up

This is the first time that my Retina MacBook Pro wakes up from sleep with a gray screen. After about 10-20 seconds, the MacBook falls asleep again.

Some points that might be relevant:

1. The only difference between power adapter and no power adapter is that with power adapter it does not go to sleep again.
2. The battery is almost full (I closed the lid last night when it was about 90%)
3. An external monitor was not attached when sleeping it, so I don't think How to fix a Macbook that does not always draw its internal screen after waking from sleep and having unplugged external display while asleep? applies.
4. I've run gfxCardStatus, almost always in "force integrated mode" (to conserve battery). And normally it wakes up correctly.
5. I run VMware Fusion 4.
6. All software is up-to-date and fully patched.

I tried to plugin an HDMI monitor and switch displays with Command-Fn-F1 and Command-Fn-F2, but that did not help (it would not switch displays).

Edit: I also tried logging in (see the comment by George Garside) but that didn't work (though the password field had focus, the screen didn't fade).

Before trying to:

• reboot (by force a power off),
• reset SMC
• and/or reset PRAM/NVRAM

What options do I have that do not loose my work?