What happens if my Campaign is reject by some display owner?

If the publisher reject your campaign on one display, it simply won't play on that particular display. Supposing your campaign is selected to play in 10 displays, and just one rejects it, it will play in the other 9, spending your balance just when the is displayed on them.  

You can see how many displays approved your campaign inside your Campaign card. There will be in this case, a number: 9/10 which means it's been playing in 9 of the 10 displays you've chosen.