If you find your cat up in a tree, they may need help getting down. Most cats can easily climb up a tree, but they do not have a natural instinct to climb back down. To safely climb down a cat needs to back down the tree (tail first) and this is a learned behavior. Canopy Cat Rescue reports that the cats most likely to get stuck in trees are young (less than 2 years old) outdoor-access cats, especially if not spayed or neutered. However, escaped indoor-only cats can also get stuck in trees.
What to do if your cat is stuck in a tree
Do NOT just leave your cat in the tree indefinitely and wait for them to come down. They might, but some will just get so weak and dehydrated that they will eventually fall out of the tree. Also, do NOT put food or litter out at the bottom of the tree. This is more likely to attract other cats or predators that will KEEP your cat hiding up in the tree.
Your cat may just be frightened and scared to come down. Try standing at the bottom of the tree and talking to your cat. Also try tapping on the tree trunk to get their attention. If your cat does not come down, you could try waiting a little while, but you definitely want to seek assistance if your cat has been in the tree for 24 hours or overnight or if they are potentially injured or in bad weather.
For more information on what to do and when to contact a professional, visit these sites:
Where to find help
Cat in a Tree Emergency Rescue provides a Directory of professional tree climbers who rescue cats. Be aware that most of these individuals do not specialize in cat rescue and may even be reluctant to do it. It is not uncommon for rescued cats to bite, scratch or pee on their rescuers! If you really want to get your cat down soon, you need to be persistent in trying to contact someone. You could also try the Fire Department, but in most locations they will not assist.
If you cannot find someone locally on the directory, then check with your local animal control, shelters, landscapers, and gardening supply companies for a possible referral to a professional arborist.
There is also an interesting article on an Arboreal Cat Rescuers Guide, but they strongly recommend that you contact a professional rather than attempting to climb up and get your cat yourself.