I have been having similar problems with my LIFX lights. With mine they are WiFi Direct and don't need a hub, so when they drop I only have to deal with that particular light. Alexa does tell me if the light is not responding and I just unscrew it enough to disconnect it from power, then screw it back in. For some reason lately they have been dropping and re-adding themselves and they show in the Alexa app with the Red or Green dot. Mostly if they go offline I am told.
I have found that there is many extraneous factors that get in the way of Alexa operating properly. One on mine was the Samsung "SmartThings" app. It saw my lights added them to its library of devices and in turn it added them again to the Alexa app. Before I discovered this I had to go through and figure out which light was the actual "working" one and delete all the others. Since I have removed them from the SmartThings app and the adding them back in has stopped. As of today none of them respond to a certain Routine that has been up and running since November, but work just fine with others. Also other devices in Routines have stopped responding too. In the Alexa Routine I can't see any disconnect. So this is going to take some hunting around.
I would imagine that your problem is that for some reason the bulb dropped from your hub and thus is marked as not responding to the Alexa app. Although your hub and bulb may have reconnected, Alexa has to "physically" go out and rediscover the bulb now that's it back. You'll probably have to do whatever "Discovery" action with the Hue hub and then tell Alexa to "Discover my devices" for it to either rediscover the bulb and that it's working now or it will add it back in to the devices list and it will deal with it on its own (as one would expect) or you'll have to add it back to the Routine and delete the non-working device.
I'm finding although Alexa is professed to work "wonderfully" this is not always the way it happens. And lately it's not working more and more . . .
Good Luck!