Most likely something is stuck beneath the key cap and interfering with the scissor mechanism. Try blowing compressed air under the key cap.
You could try removing the key cap yourself, but be warned that if you pry up the wrong side you will likely damage the scissor mechanism as they contain very small plastic studs to hold the key cap. Also, Apple uses keyboards from multiple vendors, so the procedure to remove the key cap varies with each one. The majority of the ones I dealt with unsnap from the top, but a fair number unsnap from the left side while a small number may unsnap from the bottom. Be very gentle if you try it and see if iFixIt has a guide to help you.
If you remove the key cap, then while the key cap is off test the micro switch to make sure it is functional. If the micro switch isn't reliable, then you know you need a new keyboard, otherwise you have something binding. Check for dirt, liquid residue and make sure the scissor mechanism is fully attached to the keyboard frame.
Edit: You may want to check with an independent Apple Authorized Service Provider to see if they will help you with the single key issue. They may have spare key caps and scissor mechanisms or they may be able to acquire one if necessary. The Apple Store is all or nothing.
