My unit was doing the same thing as others here and I was able to fix the issue. I believe the issue is that the sensor for the water level, which is located in a tube connecting the two water reservoirs in the main unit, has something blocking the sensor. Most likely this is bacteria growth or other debris which has built up over time. I used Oxy Clean which has some bubbling action and very hot water to clean both sides for about 30 minutes. When I flushed out the reservoir, it started working again.
If take the base off the unit by removing the 4 screws, you will see the "tube" I'm talking about. Since this has what appears to be sensors in it, I didn't want to remove the tube from the bottom to clean it and instead to the approach above. Hope this helps someone.

I experienced the same thing. It seems to take time for the water level to get to a high enough point in the base after going though the filter. After placing the water tank back on the machine, let the water "find its level" for about 5 or 10 minutes. You will hear the bubbles comeing out of the filter during this time. Then turn the machine on. This is what worked for me.

Http://www.appliancefactoryparts.com/content/pdfs/123738.pdf