why have my cherry tree leaves and buds gone brown

Yes. If you've "just" planted, my guess is you've not supplied enough water during the sunny and windy weather we've been having. It's a tree, will need a gallon a week applied around the base, twice a week if it's very hot in the summer. Keep an eye on dryness for the first year of its life - after that, it's less of a problem.

Yes, I agree wtih Bamboo. I would only add that you should absolutely deluge it with water when you water it - don't just sprinkle water around the surface. Cherries are notorious for producing shallow root systems and the roots can break through the lawn, or whatever, if watering only wets the top foot or so of soil. Water deep and the roots will stay deep down where they should be!