Bearss Lime Tree - Help

I am new to the group and i tried doing some searches but couldn't find much more info.
I have a Bearss Lime Tree that I keep indoors in my office. I have never had a problem with it and it gets plenty of sun nearly half of the day. Well for at least 3 years it has been great, but this year all the leaves fell off. The tree is still very green in most places but looks as if either it is dying or that it thinks it is winter.
Any thoughts!

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