The box to create password won't appear

I'm trying to protect a PDF with a password. When i click that option in Acrobat Pro it brings up the white box where I should be able to set the password (have done numerous times in the past). It's currently just a white box with the spinning circle as if it's laoding, but id doesn't load. Tried multiple times yesterday and today, with computer restarts in between, and it's not going away. Any help gratefully received!