What kind of tree is this? Also, what happened to it? Suddenly its leaves are...
We cannot say from your one leaf what kind of tree it is. Possibly a cherry. We'd need more information. How does it flower? Any fruit, berries or seeds? What color does it turn in the fall? Bark appearance?
We assume the leaves in the background are from the tree. We don't see any insect or disease signs on those leaves. If you see insects or disease symptoms (other than death) send us photos.
Half of all plant problems have nothing to do with insects or diseases but are environmental. Please read over the following pub for possible causes: http://extension.umd.edu/learn/common-abiotic-plant-problems-hg86
You might want to call us and speak with a consultant.