A tripped MCB, blown fuse, or a broken bulb at the start of a loop circuit are the most common reasons a whole lighting circuit goes off.
🏠Homeowner view
If one or more lights suddenly stop working, start by checking your fuse box. Look for a tripped circuit breaker (it will be in the middle position or pointing down instead of up) and switch it back on. If it trips again immediately, there's a fault on that circuit — don't keep resetting it, call an electrician. If the MCB is fine, check all the bulbs on that circuit — especially in older loop-wiring systems, a failed bulb can sometimes break the supply to other fittings further along the loop (though this is rare). If some lights work and some don't, check the wall switches too. If nothing obvious works, you need an electrician to trace the fault — common causes include a broken connection inside a ceiling rose, a failed junction box, or a damaged section of cable.