My Rowenta steam generator (pro precision) heats but no steam?

My Rowenta steam generator needs a new knob on the water resevoir. I was able to still use iron as normal by tightening the knob with a spanner. When using it the other day there was a pop, the iron went off, I changed the fuse, now the iron heats but won't steam. Where can I get a new knob or if this is not the problem where can I get it fixed?