A320 CREW HAS LEFT ENGINE GO TO IDLE FOR NO APPARENT REASON. ATTEMPTS TO INCREASE THRUST MANUALLY ARE UNSUCCESSFUL.

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L ENG WENT TO IDLE AND REMAINED THERE FOR THE REMAINDER OF FLT. COULD NOT CTL THE ENG'S RPM SINCE IT SEEMED TO HAVE HAD SOME SORT OF FAILURE BTWN FADEC AND THE THROTTLE POS/AUTOTHRUST COMPUTERS. FMC'S ALSO SENSED THAT THE ENG HAD FAILED; EVEN THOUGH IT INDEED HADN'T; BUT HAD MERELY GONE TO IDLE THRUST. AN EMER WAS DECLARED ABOUT 10 MI FROM THE ARPT. FLT ATTENDANTS TOLD THAT WE HAD DECLARED AN EMER; TO PREPARE THE CABIN FOR A NORMAL LNDG AND ALSO ADVISE THE DEADHEADING FLT CREW MEMBERS (AT THE EXITS) OF WHAT WAS HAPPENING IN CASE OF AN EVAC. NORMAL LNDG AND ROLLOUT; IN FACT THE L ENG REVERSED NORMALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATES THAT HIS FIRST THOUGHT FOR REGAINING CONTROL OF THE LEFT ENGINE WAS TO TURN OFF AUTOTHRUST. THE ONLY INFORMATION THAT THE ECAM PROVIDED WAS A MESSAGE NOT TO TURN OFF AUTOTHRUST. NO OTHER GUIDANCE WAS AVAILABLE FROM THE ECAM OR THE PILOTS HANDBOOK. THE REPORTER DID NOT KNOW AT THIS TIME WHAT CAUSED THIS MALFUNCTION.