In 2009 we had our cottage restored - it was a ruin open to the sky. We had electric underfloor heating installed. I am getting the cottage ready for someone who is staying over Christmas and the New Year and it looks like there might be a fault with the lounge wiring. The other rooms’ floors (tiling) have warmed up nicely and the lounge is still cold. The thermostat is showing four bars (ie should be full on). I’ve checked the fuse board - all fuses are fine.
The company that installed the floor has gone out of business.
If it’s still cold tomorrow morning I’ll be getting in touch with our usual electrician, in the meantime I’d be grateful if someone could tell me please, how much of a problem have we potentially got? I of course am imagining that all the tiles in the lounge will have to come up. 
It may just need flushing. We had ours done 5 years ago or so as the furthest end of the system was distinctly cooler, and it did seem to help.
It’s an electrical system, no water.
I would think you need to check the resistance of the cable.
Your electrician can easily test if the thermostat is actually pushing out power. Given the general robustness of electric underfloor systems the thermostat is always the first thing to suspect.
Next up is to look at any connections between the thermostat & the cold tail of the flooring element(s).
If an underfloor tempertaure sensor is fitted then that needs to be checked too.
They can also easily check the resistance of the outgoing heating circuit. Clearly I don’t know the area of heating that you have a problem with, but as a rough guide 5m² of 150W/m² should have a resistance of around 70Ω.
If the resistance is very high/infinity then there’s a break in the underfloor wiring somewhere. That can be traced with specialist devices, the offending bit dug up & a specialist repair can be made.
The BIG question is whether anyone has made/drilled any holes in the tiling for any reason, or had any accidents or breakages that might have damaged it in any way? If so that would be a good place to start.
Thanks very much Badger. Nothing drilled BUT we do have water ingress under the floor in very wet weather (we’ve had sockets trip in a couple of walls before now) and this winter has been (and still is) very very wet - we’ve got standing water in the surrounding fields. That said, we have had it this wet before and never had a problem with the floor.
Duh! 
We had two cold rooms (electric underfloor…) while the rest of them were ok.
Each area had a separate fuse on the box, and it turned out that the one fuse controlling those two rooms had gone…it was not apparent looking at the fuse…Electrician came, diagnosed, repaired in an hour.
Fingers crossed - if that’s the problem I’d be delighted!
Still cold this morning, so just sent distress email.
As @SuePJ has power at the thermostat it won’t be something that simple.
Hmm… Theoretically that won’t really effect the heating element, but if the damp has corroded some connections somewhere then that could be the cause.
If you have wet enough connections then one hopes that a différentiel would have tripped off
Thanks Badger. Good point, because that’s exactly what has happened to the sockets in the outside wall in the past when we had water ingress. They’ve tripped. (And not this winter so far). So hopefully it is just the thermostat.
Thank you @Badger, you were right I’m delighted to say. It was the thermostat. Changed this morning and floor heating up nicely. 
Very relieved.