as long as it didn't leave a hole that you can see light through, a tire can always be reused after a flat. any thing bigger than that will keep the tire from containing the tube fully--more flats.

even then, one can still be limped along if you have to get home. i keep some duct tape rolled up on a tire lever in my saddle bag. whenever i get a flat, i put a little square of tape behind where the hole was on the inside of the tire for some extra insurance.