somewhere, most likely close to the cylinder or the boiler, you will have a pump and a 3-port valve.
the valve receives heated water from the boiler, and is intended to open and shut the pipes which lead to the radiators and to the cylinders. Combi boilers do not have this type of valve.
They eventually wear out. If it is just some of the radiators getting warm, especially the ones close to the 3-port valve, then it is probably the rubber ball inside, which opens and closes the flow. If they all get hot fast, as if you had turned the heating on, it is probably an electrical fault, like a wire falling off, or the motor inside the head of the valve. As it is not a gas part, and is not part of the boiler, you do not need a gas qualification to change it. A competent plumber can do it, or an experienced handyman with plumbing ability. Or you can pay a heating engineer, or it will probably be covered by your maintenance contract if you have one.
The rubber ball is a wet part and usually involves some draining-down and refilling. The motor, or an electrical fault, is dry and easier to change. The motor can usually be detached from the valve and a new one fitted. If you get charged for an entire new valve, ask to keep the brass part even if it does not get fitted. You may want it another time. The valve will probably be Honeywell, or ACL Drayton. There are also some cheap brands....