Whilst the filter is off, open up the valve into a bucket and see if water comes through, it might only come out of the inlet pipe.
If nothing comes through you have lost siphon. Place a length of pipe in the valve inlet and suck, this should get the siphon action going. Try not to swallow to much of the water
If this doesn't work... fill up the canister before reconnecting and then pump the siphon pump, you should see the air being forced out of the out let pipe. Switch on and keep pumping until the air stop.
Before trying any thing make sure the ball is in the inlet pipe and not stuck.