You performed an upgrade of VMWare Esxi to a newer version and there is reason to downgrade e.g. if network drivers are incompatible, new upgrade has software issues etc
To revert to a previous version of ESXi:
Back up your configuration data before making any changes e.g. with vicfg-cfgbackup
In the console screen of the ESXi host, press Ctrl+Alt+F2 to see the Direct Console User Interface (DCUI) screen.
Press F12 to view the shutdown options for the ESXi host.
Press F11 to reboot.
When the Hypervisor progress bar starts loading, press Shift+R. You will see the warning:
Current hypervisor will permanently be replaced
with build: X.X.X-XXXXXX. Are you sure? [y/n]
Press Y to roll back the build.
Press Enter to boot.
VMWare ESXi 5.x