The message is generated by the HMI (Panel) . It means the HMI software has booted up, but it cannot establish a data link with the NCK inside the NCU.
: This is the most frequent culprit. When the CCU (Compact Control Unit) battery fails while the machine is off, the internal SRAM memory (containing parameters and PLC data) is lost. sinumerik 810d waiting for nck connection
Before diving into software resets, inspect the card behind the machine panel: The message is generated by the HMI (Panel)