My UV is not square?

I was unwrapping various geometry and somehow my UV area is now not square (but a rectangle).
How might I reset this? Will it be a problem?

I remember something about a new version of Blender having UDIM support, not sure if it’s realated (I’m a noob)

p.s .might have happened when selecting and unwrapping multiple objects (walls) to one UV

doh. OK total noob issue.
When I created a texture to bake to, I went to change it to 2048 but accidentally left one side at 1024!