What might cause a wind turbine to stop spinning?

There are several factors that could cause a wind turbine to stop spinning.

Wind turbines that no longer spin well could be affected by ball bearings or bushings gone bad (depending on the style/model of wind turbine some have bearings others have bushings), the throat collar has somehow become damaged/dented causing friction between the collar and the turbine head, or perhaps the wind turbine does not have sufficient intake air ventilation (in the eaves, overhang or low on the roof) to replace the exhaust air exiting the wind turbine.