Suddenly this morning, the UI has switched from clicking = drag the page to clicking = marquee select. But I made no changes to the default click drag behaviour in the prefs and even when I set it back it does not work.
???
You probably hit H. Hit H again. On your keyboard.
Oh, and I changed the title of this post 
Thanks!
Or you accidentally clicked one of the icons at the right hand end of the status bar (bottom of the screen) and selected the other mode.