Your Schlage lock will work from the local keypad even if your entire home automation system is not working, so it doesn’t matter what device type handler you used, you should be fine on that.
As to whether this will happen again: history tells us that it will. SmartThings has an outage about once a month, and that’s not counting the planned outages when they do Hub updates. See the links upthread for the various reporting pages.
So if you intend to continue using it, you need to have a backup plan. See the following thread (this is a clickable link)
As for me personally, as I have reported in the forum before, Late in 2015 I made the decision to move all of my critical use cases off of smartthings and onto other systems because it just was not reliable enough for me. The other systems that I use have a much smaller choice of compatible devices and much simpler rule sets, but they also have an MFOP (maintenance free operating period) Of 6 to 12 months. And they run locally without any cloud dependencies except for voice control.
I continue to use SmartThings for some convenience case notifications, like a notification that rain is expected and the guestroom window has been left open and the guest is away from home, because it does those better than any comparable system in the same price range. And I keep hoping that they will improve things. But there continue to be outages of some sort every month, and usually every couple of weeks.
That may change once they have fully migrated to the new platform, I certainly hope that it does.
But no matter what home automation system you have, you should always make sure that you have a way to get in and out of your house even if both your power and Internet are out and your home automation hub has been destroyed. Even the best system isn’t perfect.