I have a Meyer Lemon Tree that used to be doing well, but has lately been losing most of its leaves. See How to tell if indoor lemons are ready to pick for what it used to look like. I had it outside for a while, then brought it inside when it got cold. Since then, it has been not doing well. Around the time of when I brought it in, I harvested two big lemons from it. The first image shows what it looks like. The second is a closeup. There are lots of new buds---are those for lemons or for new leaves?
It is responding a stressor, like not having enough light. Less light is the trigger for trees to loose their leaves in the fall. Those leaves change colour, because the plant cut off liquid to leaf. As the leaf stop making energy it shows it real colour. The colour it has without photosynthesis.
Your plant should be at your brightest window and have a grow light. It needs that extra light to survive indoors in winter. Imagine where it is from, it gets sunlight year round. You need to provide the same. Once you get more light on the tree it should releaf.