I concur with what has already been said - the armature is stuck, probably due to moisture in the gauge corroding it.
If you're patient/careful, you can 'un-crimp' the metal bezel from the body of the gauge by carefully and progressively levering it apart with a flat bladed screwdriver. The needle can then be freed off (they are free to rotate 360 degrees in these gauges). Drop a pack of silica gel in there while you've got it open. I use a miniature bench vice (with plain jaws) to then re-crimp it all back together.
Or you could just fork out for another one!
It seems to be a bit of a design flaw that the gauge body is not a sealed unit leading to moisture ingress.