It sounds like the output to projector might have switched from duplicate view to extended desktop. Were you using Presenter View at the time (i.e. slides notes and other info display on laptop screen while slide displays on projector)?
Sometimes I have to switch this view on the fly when I encounter a glitch like this. Easiest way is the key combo "Windows key + P." Keep holding the Windows key down while you cycle through the display options with the "P" key. When the correct one is selected, let go of the Windows key. If you are using Presenter View, select "Extend." Otherwise, select "Duplicate."
P.S. If Presenter View is selected within the Setup Slideshow window, newer versions of PPT will automatically change the screen display to "Extend" the moment you run the slideshow since Presenter View can not run when screens are cloned.