Bearing have gone bad in the electric motor. You will have to replace the electric motor. They are expensive but still cheaper than buying a whole new heater.

Not spinning freely doesn't always mean the motor is bad or bushings seized. If the graphite pump impeller gets dirty or gummed up it will stick to the inside of the pump housing. It will feel just like the motor is stuck. You can remove pump housing and spin the motor, if it spins freely without the impeller installed you need to clean or clearance the impeller. If it does spin freely then you can clean the graphite impeller and inside the housing with brake parts cleaner to remove any gummy buildup that will cause it to stick. Often you will think the motor is seizing and apply a lubricant to the bushings, especially the rear, and this will creep into the impeller and gum it up. Never use an oil-based lubricant on the pump impeller. If after you clean the rotor it is still tight feeling, especially the case with a new impeller, you can sand the surface of the impeller to increase the clearance between the impeller and the pump housing. Lay fine grit sandpaper on a flat surface and massage the impeller back and forth to remove some of the surface, do this carefully and in small increments as you are increasing the clearance inside the pump and can remove too much and lose the pumping ability if your clearance is too loose. Re-install the impeller and gently check to see if the motor still spins freely as you tighten the housing. Always remember to unplug the heater from electrical power when working inside the heater and be careful with the fan blades they can be rather sharp! Hopefully, this will help someone get their heater going and not have to replace the motor!