Renault Clio T Reg Automatic?

Hi, I have a Renault Clio T Reg Automatic (2000 i think). I was out driving it today and was about to turn when it lost power, the engine was on but i had my foot on the accellerator and it wouldnt hardly move - trickled along and when i put my put down it just didnt have any impact! I thought maybe it wasnt in gear properly but it was. Has anyone any idea of what has caused this, it has never done this before. I cannot now drive it. Any help would be appreciated. Thanks