Can't turn furnace fan on from thermostat control

cnnd2010

We have a Weathermaker Infinity 58MVP100-F-17120, Series 170, manufacture date 2003.

We leave the fan on continuously and occasionally change the fan speed by cycling the fan switch from 'Auto' to 'On' then back to 'Auto'. This cycles through the off, low, medium, high settings.

We have electronic air cleaners on the furnace and yesterday I removed the cells and screens to clean them. Before I removed them, I reduced the temperature setting on the thermostat to ensure the furnace wouldn't come on and may have cycled the fan off - not sure I did that but probably did - and then cleaned the filters. After everything was put back into place, I cycled through the fan settings at the thermostat but the fan never came on. It comes on whent the furnace runs and if I leave the switch at 'On' but it won't cycle through the different speeds. The yellow LED light is continuous.

The last thing which I just tried was to turn the power off to the furnace for one minute - still can't get the fan to come on.

Help, please?