I have had a wetback installed for just over a year and worked excellent. Getting up to 75-80 degrees with solar backup. Today it stopped circulating. Fire is roaring, top pipe from wetback is hand hot, bottom pipe is cold. Water cylinder is at 40 degrees at bottom and 10 degrees at top. No sun for solar over past 3days, but wetback usually heats water to at least 60 degrees. Any ideas to fault. There is an Ajax valve fitted to the wetback loop and a vent pipe through the roof all are cold.

Linkback: https://www.plumbers.nz/q-and-a-wetbacks/61/wetback-not-circulating/1193/

Had anything done recently, eg. Drained cylinder to change an element? Any alterations to the water system? Anything that would involve turning off the water in any way?
The reason I ask, is because it has worked for a year without fault, suddenly it stops. Something has changed... What?
My diagnosis from your description... It never worked properly from the start, but the element covered the fault by keeping the water hot, now the element is blown, or the ripple controller has gone bad and no power to the element, and the problem suddenly appears...

Or I might be wrong.