This prompt indicates that the thermistor of the hot bed has detected an open circuit, possibly due to wiring problem or damage to the thermistor itself.
Check the wiring: Ensure that the wiring of the hot bed thermistor is stable. If it is loose, reinstall it.
Replace the thermistor: If the wiring is normal, but the problem persists, you may need to replace the thermistor.
Check the cables to the hot bed thermistor to ensure that they are not loose or damaged.
If cables are connected properly, replace the thermistor.
Restart the machine and check whether the fault is rectified.