Beamer250

Hi - I drove my Audi to the shops today and, just as I came to a stop, I heard a clanking sound from teh rear passanger side. I drove the car on a 300-metre round trip at low speed, and couldn't hear the sound again. When I've come back to the car, the accelerator seems very spongy, and there is virtually no power. The car will not rev past 2,000 on the move, meaning that I can only (very gradually) get it to around 60mph. There are no warning lights, and I can't see any evidence of anything wrong. I'm told that there is something called 'limp mode' which the engine mangement system may have initiated if it sensed something wrong - if so, why are there no warning lights on? Would appreciate any insight/thoughts anyone could give? Cheers Chris