When you say you can't find green in the branches, but only at the base of the tree, I assume you mean green wood. Leaves turning yellow can be due to various reasons. The most common one is a fungus disease called cherry leaf spot. It causes defoliation but rarely kills the tree unless defoliation occurs for 3 or more years in a row. Weak trees are more susceptible to cold injury and borer attacks. Borers are a strong possibility as well as any damage to the bark near the base of the tree. If the upper branches don't have green wood when you scrape the bark, it doesn't sound hopeful. I think at this point in time it's a matter of waiting to see what happens next spring - whether it leafs out and flowers. But if there have not been any leaves on it sinde midsummer, it would have no way to replenish food reserves to its root system.
