After a power outage the freezer display will typically beep and/or flash a code like PO for power outage or PF for power failure. In most cases you will need to press a button to reset the alarm. Refer to the owner's manual for specific information.
Common
02 - Freezer Door Switch
If you've confirmed there has been no interruption of power to the freezer, the door or doors are all closing properly, and the temperature inside the appliance is sufficiently cold, it's likely the freezer's door switch is malfunctioning and will need to be replaced.
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When a freezer is beeping, a fault code will usually be displayed to help diagnose the issue. Refer to the owner's manual or do an on-line search for fault code designations and possible solutions.