Justin is right. Pics would help. But consider this. Elms are deciduous trees which do drop leaves. It is a natural thing. While the leaves are off the tree less water is needed, but I don't mean 'bone dry' either. The soil should be moistened good but not soggy wet, and should dry some between waterings. Refer to the 'chopstick method', like a dip stick to check moisture level in your soil.
New buds should appear soon. As they swell and pop they will need more water, more sun, more fert. Don't fertilize in winter. Don't fertilize when the tree has no leaves. Do it in the growing season. I think your little tree has just passed through a short dormancy period/seasonal change and (hopefully) it will come roaring back.
Meanwhile - don't give up hope - read all you can about basic bonsai culture. Learn all you can about elms/Zelkova. They are fascinating. Impress your girlfriend !!
Get her a nice little ficus or azalea bonsai - don't wait for her birthday!!!