Do you know if you had the screen set up as duplicate or extended? If Extended, then Powerpoint adds a layer of complexity. It wants to show a presenters view on one screen and the actual presentation on the other.  Based on your description, you might have been in Extended mode, but Powerpoint somehow got the two screens mixed up.

I personally prefer Extended, and then make sure that I have the Powerpoint on the laptop screen before starting the presentation. I have Powerpoint set to show the presentation on the 2nd screen. Somewhere there are presentation settings you can control for this. That way, any timers and speaker notes I've added to slides will show up on the laptop, plus you get a preview of the next slide. Only the slides go to the big screen.

But, if you don't have a chance to test and set up in advance, it's best to just duplicate your laptop screen on the big screen.