If they are ELV fed by a transformer, chances are that the transformer is kaput. Otherwise, start looking for a loose connection at the switch and continue to trace from there. You will, of course, need a 2-pole voltage detector and some basic knowledge of electrics.
Im going to guess it's one of those kits with a 60VA transformer supplying all the lights.
then as one or more lamps failed they were replaced with 50W ones insted of the 20W ones youre suppoed to use, thu overloading the tranny and it's thermal protection operating.
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