Sad balcony lemon tree

Hello! We have a little lemon tree in a pot that I bought in January. It was covered in leaves and fruit, and it continued to fruit for maybe a month. I kept it inside most of that time because it was too cold on the balcony (according to the label it shouldn't be out below 5* celsius) but occasionally put it outside on warm days. It seemed very dry in early March so I watered it — too much, I now realize, and all the fruit turned black. So I went to the garden shop, they gave me some Nutrimax, which revived it for a bit. It half grew some lemons and started to bud but then the lemons just stopped growing and the buds stayed half open and all the leaves fell off. I pruned back any dead branches and took off all the dead leaves and stuned lemons, put it outside in the warm and continued to feed it with the nutrimax and water it when it seemed dry about an inch under the soil. The trunk is still green but nothing has grown for several weeks, not even branches or a single leaf. I realized the root system had outgrown its pot so I repotted it in a bigger pot on some nice new soil on Friday, but still nothing. How do I wake it up? Or is it dead? I feel so bad for it!