There are two separate issues that need to be discussed regarding this specific device.
(First rule of home automation: the model number matters.)
The first is that Sengled devices take a little longer for those zigbee checkins I just mentioned than some other brands and they tend to disconnect from the network fairly often because of that.
The manufacturer’s solution to this is to recommend that you set your smartthings account to allow “insecure re-join.“ this allows devices that have been disconnected from the network to re-join without needing the security key. Which is exactly as scary as it sounds. It’s not a good idea, it’s not recommended, but on the other hand, these are lightbulbs, so do you really care if somebody asks your network to add an extra one?
Which has a really really really long answer which can be summarized as: theoretically, there are a few people that should care, like nuclear power plants or banks or Mark Cuban‘s house, but honestly, most of the time in general there’s no practical target for a hacker, they have to be close to your house for quite a long time, be very patient, and have some reason for why they think adding another lightbulb might be useful. All of which is why smartthings still allows you to turn off secure rejoin. And which is why manufacturers like Sengled can get away with recommending that you do so without putting a lot of black box warnings around that recommendation.
But understand that once you do that you are doing it for your entire zigbee network. Not just one specific device. It’s like leaving a second story window unlocked so that your teenager can come in that way without waking up the dog. It’s not necessarily only going to be your teenager. But then, from a practical point of view, how many people are even going to guess that you left that window unlocked? Or be willing to claim all the way up to use it? Most serious burglars will just break a ground floor window If they want to get in, right?
So… if you are willing to leave yours zigbee network running in insecure mode, then you can follow the Sengled recommendations.
Once it’s been added, then you should be able to use smartlighting with it just as you did, so I agree with @garrett.kranz , you likely just ran into the platform problems today.
There have been platform problems four or five times in the last six weeks, unfortunately they are not uncommon.
You can sign up for outage notifications on the following page, although not all glitches get reported there and sometimes they don’t get reported for a couple of hours.