Some community members have reported similar issues. I’m sure it’s incredibly frustrating. 
Much of the time it appears to be a cloud glitch. (smartthings is still a highly cloud-dependent system.)
First thing is to check two different means of accessing your cloud account to see if the sensor shows up there.
The official web Ui is at
There is also a very popular community built web Interface, which actually shows more details:
SmartThings API Browser+ ... Now Available to All
In both cases at this point you’re just looking to see if the device is still present through one of those methods.
Once you know that, open a support ticket with official smartthings support.
The first person you get will probably just be a general Samsung employee, working off a script, with little or no knowledge of smartthings architecture. They will ask you to do all the usual tedious stuff, probably including removing and reinstalling the app. You can do anything they suggest as long as they don’t suggest a hard reset of your hub, because that would require rebuilding everything and likely is not really necessary.
If they can’t solve it, and you hang in there, your case should eventually be escalated to actual engineering staff. They have access to a lot of information that we don’t, and they may be able to figure out what’s going on.
Good luck— let us know how it goes.
Oh, and zooz tech support is usually both knowledgeable and helpful— you might want to contact them as well.
I myself was a field tech who had worked with both zwave and zigbee before I ever got smartthings for my own home. I find the absence of the typical Z wave diagnostic tools pretty frustrating, but it is what it is.
(I have a thread where I have previously noted some of the gaps in the smartthings Z wave implementation, if you’re interested. But none of that is likely to help your immediate issue.