The dimmer could have failed. The supply could be lost on the circuit upstream of the dimmer. The neutral could be lost to the lights that do not work. There could be a loose connection on the line or switch line at the light.
etc etc etc
Are you able to safely test for output at the dimmer and/or voltage at the light ?
EDIT: If the feed line is linked at the dimmer and one side works then I would suspect the dimmer probably due to failed lamp(s) or it could still be a loose connection at the lights in question.
If you feel confident you could try swapping the cabling on the two dimmers round. if the other one now doesnt work then the problem is somewhere else. if it is still the same dimmer faulty then there is your error. swap out the module in this case
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