Q 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? by Bob Pringle

A 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.