Empty it out first then take out the intake, remove the impeller and clean it out of any gunk, take an old toothbrush and run it over the while thing also making sure there is no gunk inside the housing, put it all back together, fill it up to the top with water and plug it back in, it will take about a minute or so to get prime back and you may need to add some more water to it to get it going but then your good to go.
My emperors were notorious for this so I would do this about every three months as a routine maintenance and they lasted for years.