Richard

Egle,

You are brave to grow Citrus in your location. Although I have never been there, I had two students in recent years from a region north of Vilnius, a broad valley as they described it.

If green leaves are falling off and the fruits are turning black then the tree is nearly dead from a fungal infection in the roots. You might spend less money obtaining a new tree than trying to save this one. However, if you wish to try:
Remove all the fruit.
Keep the temperature of the pot and the air above 15 C. For air temperature above 25 C in the house, make sure the air is not too dry.
Give the plant at least 8 hours of "daylight" per day. Two florescent bulb fixtures with 2 1-meter bulbs in each would be about right. Position them about 1 meter above the plant but not directly overhead. Instead place them about 0.5m on each side. Use a bulb that is rated for "daylight" or a "plant light" if you can afford it. Do not use a "black light" bulb. If you use incandescent bulbs, place them higher -- about 1.5 meters so the heat from them does not hurt the plant. This lighting should be enough for several months but is not a substitute for real sunshine in the summer.
Apply a horticultural copper solution to the soil. A professional florist or a nursery that raises flower bulbs can help you with this. If you want to make it yourself or you know a high-school or college chemist, apply 1 liter of a 1% solution of copper sulfate that has been buffered (perhaps with gypsum) to keep the pH around 6 or 7.

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