Just recently I modeled a small three drawer jewelry box and square corners appear rounded! I am a handicapped user and use a touch pad mouse with four buttons. I am constantly hitting the buttons and I wonder if, upon, accidentally hitting a button I possibly clicked on something and changed a setting?
As @MikeWayzovski pointed out, without seeing the model we are kind of guessing, but you could try triple clicking the model (that will highlight all connected pieces) then, in Model Info, turn off Soft and smooth.
Thank you for that info however, when I highlighted the whole model I went to the model info window and I couldn’t find where to soften edges. So, with the whole model highlighted, I went to view, edge style, and clicked edges and what do you know it fixed my problem! I have no idea how I got that to start with but problem solved.