help diagnosing the plant's disease needed

Something has happened to our house plant... its leaves suddenly started becoming yellow - and very quickly the whole plant is becoming so. Just a month ago it was slowly having new leaves and then it stopped... the new leaves which have formed have stopped growing as well...
In our previous apartment where this plant was really blossoming it was much less light. Right now, we have a lot of sun but I tried to move the plant into the area where there is little sun but it seems not to help :((
Would anyone be able to help me with this? Would be so sad to loose this plant...
Thank you! All the best Egle
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