Two ways.
1. Back feed with mains pressure, ie connect a hose from a mains tap to one which is fed from the tank - this will push the airlock back into the tank.
2. Use a wet vac on one of the tank fed taps, this will pull the airlock out of the tap. This method is easiest. Create a good seal between vac hose and tap using a damp rag, keep sucking until your vac is full then pull the hose away and water should continue to flow. May need to do it a couple of times if it’s bad.