You have not provided any exact system info, no version info nor any other details, so it is impossible to make sense of thsi. From hardware acceleration issues to mundane problems with input devices this could be anything.
Are you getting an error message then? If yes, what does it say _exactly_ or could you paste a screen shot here.
What version of Photoshop are you using?
What operating system?
What have you tried in order to fix your issue? Have you tried resetting Preferences for instance? We need a lot more information to give you a decent answer, but from the little you have told us, my gut is that it's either user error, or will come right by resetting Preferences.
Thank you. Got it fixed. No error message — just wouldn’t for a rectangle for cropping. Went into the Preferences somewhere and hit “Preset” and that seemed to be what got it working again. Also neutered something where my shift key could change modes. Not sure what — but after a couple of frustrating hours (including this post and some unhelpful tutorials) — it is working again and I am back on my project.
If I post here again I will be more precise about my operating system (mac) and version of PhotoShop (current, rented monthly from a cloud).
Thanks again for the responses!