A few possibilties here - either the plumber knocked something off, or you had a frost that damaged and drained the system, possibly the pump has seized, or any one of a few eletrical things initially spring to mind.
Most tube systems don't have a heat dump when they should have. This means that summer the system overheats, damaging the antifreeze. However, usually there would be a phase during which you would hear air gurgling in the pipes.
You need to look at the controller and see what temperature your panel is reaching. It should usually be 6 to 10 degrees hotter than the bottom of the cylinder. If the panel is at 80 and the cylinder is at 20, then check the flow meter and see if the pump is running.
On the other hand, if the controller is making no sensible readings at all, you may have a damaged probe, bad connection etc.
Either way, I think unless you are a very competent DIYer you're going to need to call out a solar installer to remedy the situation. PM me with your approx location if you want the name of someone in your area.