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I assume the Smart's battery is fully charged.
If the fob has been causing problems recently, you can assume an electrical problem with the fob's circuit board. Maybe you could open the fob and see if there's a cold solder joint somewhere on the board.
In the worst case there is no other choice but to buy a new fob.
The fact that the door locks cannot be turned is probably because they are jammed because they are not used enough and have not been lubricated. This is often the case with vehicles with fob because the locks are never moved.
But even if the locks were to rotate, without the Fob signal, the immobiliser would not release and the car would not be able to start anyway.
Either you use the second key (if available), check the solder joints, or unfortunately you have to drag the car to Smart to get a new Fob.