Signs of green thumb absent

Almost overnight, the lemon tree I have cultivated carefully from seed decided to shrivel and die, dropping all its leaves and generally looking as sad as any plant I have ever seen.

Apparently leaf drop is common - but how this has happened is beyond me. Mrs. Ottawacker - not a staunch proponent of lemon trees in the bedroom - is mysteriously avoiding my eye... so I am beginning to suspect an outside influence. But poisoning is not her style. 

Anyway, I have pared it back and already there are signs of regrowth... Might be time for me to invest in a cactus.