Emperors are bad about not priming, especially if you let the water level get low.
So either it needs cleaned, or your water level is too low, or both.
Clean it, especially the impeller and put it back together. Make sure your water level is all the way up near the rim.
Fill the filter with water and start it. If it still has trouble try lifting the intake tube up and down a few times and seating it well and it should restart.