Interestingly, Bonsai, literally translated, means �tree planted in a tray" and almost any tree, properly cared for can become Bonsai.
Most tend to think of Bonsai as evergreen varietals. However, many decidious trees are successfully transposed to Bonsai. Deciduous Bonsai go through a dormancy stage at winter and I suspect this is what has occurred with your plant. If it is (was?) an evergreen, however, you should perform Matsugo-no-mizu and kiss it goodbye.