I had this with mine.
The tolerances between the arms are quite fine, and there is only a wafer thin bushing between them, so any muck or corrosion can cause them to seize.
First thing to try is to blast the inside of the brake with some brake cleaner, let it dry and then blast it with some spray lube (WD40 would probably be OK in this instance). If that doesn’t free it then you’ll need to take it apart to clean the surfaces.
Check the park tools website for a good guide on how to do this, be warned though, its a little more fiddly than you might think, when you put them back together you need to ‘preload’ the bolts that hold them together (similar to cup and cone hubs) which can take a few tries.
In future, if you ride in the wet, spray them with lube or something straight away, otherwise they’ll be seized next time you take the bike out!