Since upgrading to Yosemite, the screen on our PowerPC desktop Mac is totally black and the light on the bottom right of the monitor flashes 'dot dash dot' repeatedly. We can't seem to find a solution having tried the obvious e.g switching off and on again. Please can anyone help?
PowerPC based Macs are old, too old to run Yosemite by a good number of years so I'm going to guess that you have a MacPro and an Apple Cinema Display in which case:
Edit: I should expand on my rather terse answer. I think that the timing of you upgrading to Yosemite is a bit of a red herring. The problem according to the KB article seems to be to do with the power supply. It may have been damaged in some way (or maybe just 'got old'). Check everything is plugged in snugly and that nothing is shorting (no hissy-poppy-crackeling sounds). Also check for excessive heat coming from the power adapter. It shouldn't be too hot to hold, if it is it implies some form of damage.