Black screen iMac lockout

At seemingly random times, the display of my iMac (21.5-inch, Mid 2011) goes completely black, and the system is then unusable, so I have to perform a forced shutdown using its power button. This iMac has a Radeon HD 6770M graphics card. Why is this happening? [ BOB PRINGLE ]

Ian Sleightholm replies: Given its vintage, a dying graphics card or other hardware could be to blame. Several models of graphics cards have suffered premature failure, probably because of the introduction of lead-free solder, and some have had their warranties extended as a result. Before you take the Mac in for repair, check its logs in Console. After this next happens and the Mac has restarted, open Console and search for BOOT_TIME, which should take you back to the last startup. The entries right before that may give a better idea as to what went wrong at the time of the black screen. Otherwise, run Apple Hardware Test or Diagnostics; if that reports a hardware fault, you’ll need to take the Mac for repair. If the hardware checks out OK, the Mac could still have a graphics card problem, which may only be possible to fully diagnose by Apple’s in-house tools. The Mac may be inadvertently trying to enter Target Display Mode (as when you press CommandF2). You might stop that by temporarily restarting in safe mode, or by checking the settings in the Displays preferences pane.