Jura Air intake nozzle

If your milk is coming through flat with no froth, this is the most likely cause - if you are only getting steam and no milk at all you more likely have a blockage on the milk side and no steam, there is a more severe problem probably internal to the machine.

This will make your milk frothy again, but to do that the milk must already be coming through, indicating both the steam and milk sections are clear and operational.