Quick answer
You have a split-load consumer unit with two RCDs — one has tripped. Reset the tripped RCD and investigate which appliance or circuit caused it.
Modern consumer units are split-load boards — they divide your circuits between two RCDs. This means if one RCD trips, only the circuits it protects go off — typically either the lights or the sockets. This is by design — it prevents a single fault from cutting all power to your home. Find the consumer unit, identify which of the two RCDs is in the down or middle position, and reset it as normal. If the half-house loss of power happens repeatedly, there is a fault on one of the circuits protected by that RCD — work through unplugging appliances to identify it.
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