Why have the radiators downstairs stopped working and how can I fix them?

The radiators downstairs have stopped working. Upstairs still come on in line with my timer controller (Honeywell) but a few days ago, with no warning, all the downstairs radiators stopped coming on. We have a conventional boiler and both pipes coming from it feel hot. We have 2 thermostats (both Honeywell) - one upstairs and one downstairs. Both click when you turn them and this can be heard at the boiler (a Glow-worm Space Saver in the garage). We've bled the radiators but all seemed to be OK.
Any ideas very welcome - we're getting very chilly!