Timed out waiting for forcewake ?

Upon startup, my laptop will display the message:
doing fast boot
Creating device nodes with udev
2.503996] [drm:__gen6_gt_force_wake_mt_get] ERROR Timed out waiting for forcewake old ack to clear.

It then proceeds to prompt for login on a black screen (not the desktop, which I was expecting).

I don’t recall changing anything recently that would cause this (though obviously I did). Any ideas on what is causing this and how to fix it?

Any help is much appreciated.

What video??

Maybe a kernel update. which may require a reinstall of the drivers, depending on how they were installed.

Video? Do you mean video card? If so, its a Radeon HD 7600M card.
Also, which drivers do you mean? The video drivers?

Yep video this kind of problem is often (but not always) video related. First place to look