rubber plant needs help

Hi,
I have beautiful rubber plant which is around 4 ft tall and was growing very nicely. I changed its soil ..put new potting soil and after that its leaves started turning yellow and stared falling off...more than half the leaves are gone ..please suggest what went wrong ..how can I revive it.

Most Ficus (Rubber Trees are Ficus elastica) do not like changes.
If it is still in the same location:
Is there a draft from a heater (that was not on earlier- before you transplanted it)?
Is it near a window that might be too cold?

Is the new soil a material with good drainage?
Did you disturb the roots when you transplanted it?

It is in the same location. I washed the roots and then put the potting mix. initially it was sitting in the water but as soon as I realized it ( around a week). I drained the water since then I didn't water it which is around 3 weeks but since then I can't see any improvement.

I wonder if that sort of transplanting (removing so much of the original soil) was too extreme.
Certainly sitting in standing water for a week is bad news. Good that you have corrected it.
Hope it recovers for you.

I wish you would have asked about the repot before undertaking it. I'm not saying that to chide you - just that we could have advised you that repotting a plant about to enter a period of quiescence (very slow growth - the plant sort of resting until days get longer) shouldn't be repotted. The act of repotting is a very good thing for rootbound Ficus, just that the timing was off. The entire Ficus genus, except for the hardy fig F carica) should be repotted in the summer. The best time is the month before its most robust growth, which would be around Father's day, for you. If you weren't careful about keeping all the roots wet during the repot, you probably lost a large fraction of the important fine roots to desiccation, which is a significant setback.

If you take the plant's natural rhythms into account and repot in summer when the plant is at peak vitality, the plant will tolerate bare-rooting very well. I've bare-rooted Ficus so large it took 2 good men to tip the pot over - probably 100 gallons + of soil, removing more than 3/4 of the roots, half the top growth, repotting into a pot half the size of the original .... and the plant resumed growth within 2 weeks and acted like nothing happened.

What you DO have going for you is, you said the plant was growing well. Assuming the plant was healthy, it should recover, given time. The key is to keep the soil warm and the soil damp - DAMP being the operative word. With leaves falling, the plant will use very little water. It's likely that even those leaves that haven't fallen are forming an abscission layer at the base of the petiole (leaf stem) and will fall soon, which means even they are not using water. Your main job at this point is to make sure you never let the soil get soggy, and wait. No fertilizer is required until you see sure signs of new growth.

If you like, I can post an article I wrote about caring for Ficus in containers. It has a lot of useful information.

Al