Finding Missing Units
If a Logger module suddenly disappears then the address of that module has probably been changed. This may be due to an electrical transient, in which case the address has probably been reset to 16.
Use the VT Setup software to find which module has lost its address. You may need to disconnect all the Logger modules and reconnect them one at a time to identify the correct one. Having found it you can use VT Setup to re-enter the addressa of the module.