Xiaomi/Aqara devices are a bit picky: they only like certain repeaters, and if they loose contact with one, they may just "fall off" your network. (Theoretically, they should be able to find another repeater or your hub directly, but ones I've looked at seem hesitant to change routes and would rather fall of my network than do so; others have said their experience varies from mine.) With "normal" devices, this isn't really a concern, but since these aren't quite standard, I'm not sure there's any good way around it. You'll likely have to re-pair them.
For regular use, if you haven't read this thread, it's pretty much a must-read if you use these sensors on Hubitat (or really any ZHA 1.2 hub):
But if they loose connection to their repeater, which sounds like happened here, all bets are off anyway. 
You just might be able to make it a bit better by using only "compatible" repeaters throughout, as discussed in that thread, if you aren't already (but again, mine don't really seem to care).
Otherwise, with standard devices, this really isn't a concern; they should re-join your network on their own in the event that they were not able to maintain a route in the meantime.
