Looks like a spring-loaded pin that has been bonked with something that damaged it's outer ring, trapping it's pin down in it. The one next to it looks like it should still operate normally.
If you are careful you may be able to use a good dental pick to work lightly, but repeatedly, in a circle around the gap between that pin and the ring around it, until it wears enough of the bent material of the ring away to let the pin pop up again. Being too rough could break the mechanism of the pin...so it will take a fair amount of patience to try this.
Mouser, Grainger, or McMaster-Carr may be able to help you find a replacement spring-loaded pin, if you contact them with the info and images. They won't likely know anything about A2B or the use to which the pin is put, but they may carry a compatible pin.