A while ago a number of MacBook users noticed a pretty annoyingon their systems, in which the computer would appear to be running but would not show any output on the display. To get around this issue, people were forced to restart their systems, losing any unsaved data.
Investigations of this bug showed the problem appeared to be rooted in the handling of the dual graphics cards inmodels from 2010, and while Apple to address the problem, this update did not help all who were experiencing it.
The primary workaround for this problem was to use the third-party GPU manager utility gfxCardStatus to force the system to use only one of its available GPUs, to avoid the automatic switching between the graphics cards that was leading to the problem. While the increased use of the more powerful dedicated GPU drained the battery a little more, it did avoid the problem for many.
Unfortunately, while Apple claimed this problem was software-based, many users reported that replacing the motherboard with a new one of the same model fixed the problem, which suggests it was rooted in hardware. Installing Apple's software fix for the problem also produced mixed results.
Some may still encounter this problem periodically on these older MacBook Pros, and it's possible configuration issues could lead to something similar on other systems. If this happens to you, try the following as a way to save your work before restarting.
MacFixIt reader Joe recently wrote in regarding his experiences with this problem, and found that while the display remained black, the system did appear to be running just fine in the background. Not wanting to interrupt his work flow, Joe attached an external monitor to the system, and despite the black video output on the main display, normal video output appeared on the external monitor. Thus he was able to restart the system properly.
Hence, if you have a MacBook (or any Mac for that matter) that is showing no video output on one display source, especially a built-in display as on a laptop or the iMac, then try using a secondary video output, if possible, before resorting to rebooting the system. With DisplayPort adapters you should be able to use most video devices to do this, including any external monitors, TVs, and even an iMac that supports target display mode.