Quick answer
Switch the tripped RCD lever firmly upward to reset it. If it will not reset or trips again immediately, do not force it — the fault is still present.
Find the consumer unit (fuse box) — usually under the stairs, in a hallway, or in a cupboard. Open the cover and look for a switch that is in a middle or down position — this is your tripped RCD. Before resetting: unplug appliances on the affected circuit to reduce the chance of immediate re-tripping. Then push the RCD lever firmly upward to reset it. Some RCDs have a button that needs pressing before the lever will reset. If it immediately trips again, do not keep forcing it — the fault is still there. Leave it tripped and call an electrician.
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