Also, the impellar that drives the filter may be gunged up with muck and debris. Dig out the manual for it and look at the diagram.
Unplug the filter. And work quickly.
You can pull the top half ( the bit with the motor ect ) away from the perforated section where the sponges fit into. The join is just above the little blue flow control knob on the front. Careful, it might be a bit stiff but it does separate.
When you have the two halves, just lift the impellar out and give it a good wash. I also get cotton buds and clean out any gunk around the steel impellar shaft ( little steel rod the impellar lifted off ) Just get in there and scoop it all out from around the rod , give it a blast under the tap ( only this bit, not your sponges obviously ) . Then put the impellar back onto the shaft, it's magnetic so it won't fall out. Push the two halves back together again, put your two sponges containers back together and put that back in . Submerge the filter and plug it back in. It'll gurgle and rattle for a minute or two but the water should start coming out fine.