Thank you aurorafrog.
Yes I did the same. Builder asked me to contact manufacturer (although one of guys at builders office did say he would organise a plumber). Manufacturer said they would need a serial number (even if the system is 2 week old/new). After they get the serial number they organise a technician to be there (you have to be present... timings 9 to 5 Mon to Fri... basically day off of work).
The builder also asked us to check the gas pilot light (because of high winds in the last 2 days). But we discovered that there is no pilot light on these new solar hot water units.
After we got home in the evening, all we did was to restart the system. Turned the power off, turned the gas supply off, then power ON and then gas supply ON. And it worked. The system started working again.
Hope this helps others.