Its cold and we have no heating, what can we do.

Sometimes there are a few basic checks that can be done.

Check 1 - Check The Power Supply.
I had an Electrician in my house and a mistake was made which meant that live and neutral were reversed. Because of this the Ravenheat CSI120 boiler failed to light and had an error code F01. Reversed the wires in the plug top and boiler was happy again.
The fuse may have blown in the plug or the spur - worth checking.

Check 2 - Check The Gas Supply.
Some customers have a pay as you go card for their gas. I have had a customer whose Baxi Boiler had an error code 7 or 9 - no ignition. Found that there was no gas at the appliance. The customer assumed that as she had £6 worth of reserve gas, that there was gas. On her particular meter she had to activate the reserve gas.
Once that was done, hey presto the boiler was working.
It would also be worth checking that the gas meter ECV (emergency control valve) has not been switched off - it does happen.

Check 3 - Check the water pressure
This information applies to combination boilers and boilers on a sealed system. Inside the boiler is a sensor which will not activate the boiler unless the water pressure is at 1Bar. There is usually a filling loop under the boiler, to top up the pressure if necessary. If the pressure keeps dropping indicates a leak and the cause would need fixing. However if the pressure has dropped a short term fix is to top up the pressure.

Check 4 - Freezing Weather check condensate.
This applies to condensing boilers only. These boilers produce a water vapour (condensate) which needs to drain away from the boiler. As this water is slightly acidic, it is drains away in 21.5mm plastic pipe. This needs to be insulated or IT WILL FREEZE. To Defrost carefully use a hair-dryer or hot water bottle.