What do I do if my viessmann vitodens boiler 100 is showing 0.5 bar ? help, have no hot water or heating now:(?

The boiler has shutdown due to low pressure.You need to raise the pressure to between 1 & 1.5 bar. There should be a filling loop near the boiler [a short stainless steel flexible pipe] open the valve to let in more water to raise the pressure.