Have you, or someone, turned the lights off at the mains, when we installed smart bulbs in ceiling lights we kept switching them off by the switch out of force of habit and then the next time you ask Alexa they wouldn't work.
No ,nothing turned off ..Alexa just says the lights aren't recognised..tried uninstalling lights & asking Alexa to discover them again but no good..someone told me a virus might have got into the network,but can't see how that would work.