Issue Description
The bare metal that has been normally activated and used cannot be connected to the server after you upgrade the CentOS system kernel (NetworkManager is used before the kernel is upgraded).
Possible Reasons
NetworkManager is not set to automatically start upon startup after the kernel is upgraded.
Solutions
Contact the operation administrator and log in to the server and check whether NetworkManager runs properly.
# Check whether NetworkManager automatically starts upon startup
systemctl is-enabled NetworkManager
As shown in the figure:
Wherein, disabled indicates that the NetworkManager service is not started.
Execute the following command line
systemctl start NetworkManager #Start the NetworkManager service
systemctl enable NetworkManager #Set to automatically start upon startup