So this week I ran into a situation where I had a bunch of the D-Link DGS-1005D 1GB switches all mysteriously turned off on me at the exact same time. So I looked into it a little bit further and discovered that if the electricity supply surges downwards at all, the DGS-1005D (and DGS-1008D too apparently) basically turns off. You may still see the power light on but likely the link lights will all be off. Simply pull the power, put it back in and you should be good to go.
This happened to me earlier this week. I do also have a DGS-1008D but it remained on, however apparently this unit is also susceptible to the same power fluctuation problem.
At this time I am unaware of any firmware upgrade to the fix the problem but I have a sneaking suspicion that the problem is in the power supply itself and not the switch component.