I had this. Turned out that water had got into the sealed sensor unit. Had to be replaced entirely - at a total cost of €80 including the price of the new LED lamp, and the electrician's labour installing it.
If you are anyway competent in DIY, then it's an easy job. Isolate the mains before starting work. In your case, it will be easy verify that you have done so, as the light will go out when you turn off the circuit. If you are not sure, then best get an electrician, they're cheaper than funeral directors.