I had my house for sale last year and moved so I did not see the tree throughout the year so something may have happened I missed. This tree is in Russellville TN and is at least 6 feet tall and has bloomed beautifully every year and always looked healthy when I saw it. This year, still no leaves at all, and has not bloomed... no sign of life.
How do I know if it is hopeless (besides waiting) and what could have happened?
That plant is gone to the great landscape in the sky. You may see a sprout or so from the root system, but it's entire functional value is lost.
Plan now for replacement. As far as what happened? There are myriad reasons for an ornamental cherry to die, most of which require some diagnostics. If it matters greatly, have your county extension agent visit or use that service to take samples for examination. The range of insects, diseases, climatic influences, mechanical damage, etc. is so large that a single picture won't resolve it.
Since you weren't even there to relate anything about it, the questions can't even be asked.