If you see an error saying 'Programmer is offline', then the system cannot communicate with the Programmer in your home. As your Programmer also holds the wiring that connects the whole system together, this will prevent your heating from operating.
Many common issues can be sorted by restarting the device. Turn off the Programmer and check that the screen remains off. To turn the Programmer off, you need to switch it off at the mains. There will be a fused switch somewhere near the Programmer, that looks something like this:
Once the Programmer is fully shut down, turn the Programmer back on at the mains. Wait for the Programmer screen to show again, then check your Passiv app to see if the error remains.
If this does not fix the issue, then you will have to contact your installer. As your Programmer is mains wired, it is important that any repairs are undertaken by a trained professional.