So mine stopped working, too. Almost certainly due to battery.
Replacing the batteries did not help a lot -- it did indicate I needed to redo the setup. Which I then did to no effect.
Per other post in the forum, this should be fixable by disassociating the Thermo completely (from account and iPhone), then redoing setup. However, any time I try to do the disassociation in the Thermo app, I get an error message "Something went wrong", "An error occured (2557)". And as such I cannot remove the device to get it re-setup.