I wonder if it's a stuck or failed diverter valve. There could be a manual lever on the diverter valve that you can move and latch in place. If that turns the heating on, it could be that the diverter valve motor bit needs replacing, rather than the entire valve.
You'll need to do a bit of sleuthing and browsing to understand more.
Fghi
October 2022
Automatic Answer
We may have found other answers which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.