Unit experienced a hard restart due to a power outage or something of the like and then the unit boots into an older version OS than what was previously running on the unit. Manually restarting the box with the defined previous OS still doesn't load the desired OS. Restarting the unit with the defined OS varies per unit. Refernece that documentation as needed.
In these cases it is a possibility that the original OS running on the unit experienced some form of corruption due to the abrupt power loss or restart, resulting in the unit no longer being able to boot to that image.
1. Delete all images except the current booted OS. In most devices this can be done via the GUI and CLI alike. Refernece the device specific documentation for this process.
2. Then download a new image of the desired OS and install this fresh copy of the OS to the unit.
3. Restart the device and the unit should boot into that freshly installed OS.
4. If the the issue persists after all the above steps have been taken, open a case with Symantec support to have the issue looked into further.