The problem doesn't appear to be at the doorbell end - That is all low voltage wiring, that is fed from a transformer somewhere - either in the consumer unit, or in a separate enclosure near the doorbell mechanism...
It's possible that the feed for the lights passes through the transformer mains voltage side first and that a wire has come loose there, but if you haven't touched that then it may also be somewhere else and just a coincidence that it happened while you were doing the doorbell.
How are the living room lights switched? It may be worth removing the switch to check for any loose wires (turn the breaker off first).
Other alternatives are a loose wire at one of the existing lights - in this case it would be at the first one in the chain if none of them are working....