Has happened to me a couple of times (I use that function a lot).

A “Reset all tools” fixed it one time, but the next time I had to reset/trash all the preference files.

The preference files are notorious for getting corrupt after time, and causes PS to misbehave.

The easy way of doing a reset is to hold down Ctrl+Alt+Shift (Windows) or Command+Option+Shift (Mac) immediately after (or while) launching Photoshop.

 A prompt will ask if you want to delete the preferences file. Click Yes.

Hope that does it for you,
Chuck