The ground floor rads do not heat up but the top floor rads are all working fine:

Is this a steam heating system or a forced hot water heating system? My bet is that it is a forced hot water system with a minimum of 2 zones....or two thermostats.

Is this a new issue? Have the radiators been working fine all winter and just cut out on the first floor now? Let me know.

Start with easiest solution... bleed the radiators. Take the air vent/bleeder valve located at the upper right or upper left of the radiators, open them and see if lots of air gets released. If it does, let the air run out and collect about a cup of water from each radiator after that. If air is the issue that should fix it.

If that doesn't help then pop back and let us know more about number of thermostats and if you have ZONE VALVES (see 1st 2 pictures) or PUMPS (see picture) at the boiler.. ok?

Thanks...

MARK