I have just disconnected a wall heater in the bathroom and have connected wires to FCU ready to connect to towel rail when finished decorating. Since I've done this the light in the bathroom will not come on.
Heating and lighting should be independent of each other. But once a DIY'er is let loose in a house one can't assume they have kept to these rules.
Your only real option is to call someone in to inspect the installation and see what is going on.
On a new house or one where the electrics have been altered this year I would look for an RCD tripped but lights in a bathroom being protected by a RCD is rather new and as yet not very common.