How can I fix my steam iron?

When I press the steam button on my iron it gurgles for a minute then the steam comes through and the noise goes away, however, now when I press the steam button it just gurgles for ages and no steam comes out. I have not put a new filter in for a while, but it sounds like something is stopping the water coming through, what can I do? It's a blue kenwood with a handle and underneath it has written, 'Model G-1033 STB-E' and 'GY0722', I don't know if this helps.