The only things I can think of are overwatering, frost damage or fungal attack. Maybe if its 15 years old it is just time for it to go to that great greenhouse in the sky...
as long as you have checked the roots for vine weevil and mealy bug - but it doesn't sound like either of those.

Hello...Its kept indoors so wont be frost, overwatering..maybe? Fungal?? not so sure coz one side of the plant is lovely & healthy. Thanks for your advice :)

Quick, take some cuttings from the good side. Money plants root easily, so you could have all the fun of starting again if the parent goes to the great greenhouse in the sky. And if it recovers, you'll have some handy presents, or things to sell at the next charity coffee morning or other suitable event.

These plants do have a tendency to succumb to a form of foot and butt rot, where the stems go mushy at the base - usually caused by overwatering, stagnant air. I'd remove all the growth down to the base on the dodgy side, and check the roots beneath too - might be best to cut the plant in half, keeping the roots and stems which are healthy on one side and discarding the unhealthy portion, preferably with a clear margin of healthy growth attached. Either that or take cuttings, as Beattie suggests. Or both.

Thank you all for your comments :)