DIAGNOSIS The 5-way joystick/navigation button is physically stuck or has debris preventing its movement. It is not a software issue.
FIX One: Ensure the monitor is powered on. Two: Apply firm, direct inward pressure on the joystick button for 3-5 seconds to trigger a potential reset. Three: With the monitor powered off and unplugged, use a can of compressed air with a narrow straw attachment. Direct short bursts into the small circular gap surrounding the joystick base to dislodge any debris. Do not insert any tool into the gap. Four: While applying the compressed air, gently wiggle the joystick in all directions.
IF THAT DIDN'T WORK The internal micro-switch may be damaged. As a temporary workaround, you can control most monitor settings via the Samsung Display Management software on your PC if connected, or by using the physical power button to toggle through input sources.
HEADS UP Do not use excessive force or lubricants. This will void the warranty and likely cause permanent damage. If the button remains physically stuck, contact Samsung support for a repair assessment; the entire rear control panel may need replacement.