Vaillant wall mounted water heater - VU-S?

The system stopped working and I found the water pressure to be at 0 bar. I increased the water pressure to 2.5 bar - and the restarted the system - and it would not re-start. I have taken it apart to check for loose electrical wires - but did not find any. The green light is on (the red light is not on). The switch or ignition will not "click, click, click" to try to light the pilot / gas. what can I try next?