Hi Cindy and Dale,
Joe R made a similar reference. I had wondered if something hardly noticed like a thin twig, lodged itself up there. The fan will collect debris when the cap is wide open, especially if not running on a higher speed. It is one reason why I don't like to leave either vent wide open when unattended for extended periods.
I would start troubleshooting by going on the roof and spin the fan by hand. Make sure it spins freely. If not, inspect for something like a twig to even something wound around the shaft of the fan, check for clearance between the blade and everything else.
If the blade spins freely, then you can look into the next possibility. Maybe the switch that turns the fan on/off when opening and closing the cap has malfunctioned. I am not sure where that switch is located in our fan. If you can get to it, make sure it is aligned right and clicks on/off when the cap opens and closes. Maybe the switch is good but got loose and is not activated.
Oops, I see keelhauler just beat me with a similar recommendation about that switch. Make sure it is working, one way or the other.