probably a loose connection in a ceiling rose.
lighting circuits are arranged in a string, usually running from one rose to the next to the next. The fault is most likely either in the last rose that works, or the first one that doesn't, in that string.
There is also a possibility that you have a fault in a cable where it has been nailed or gnawed, or in a junction Box (if you have such a thing) hidden under the floor,. this will be more tiresome to find, which is one reason why you are not supposed to use them.
Do not disconnect any wires from a ceiling rose until you have labelled each one, and sketched or photographed it so you can reconnect them in the same way. Lighting roses contain several wires of the same colour that do different things, so you must not mix them up. Isolate the power before starting work. Lighting roses are live even when the light switch is off, and sometimes even if the circuit is not working.
Go and invest £10 in a multimeter. a neon screwdriver will not do. Also a permanent marker and some white vinyl tape for labelling the wires.