Carefully pop it off to see if there is any visible problem. (You might have to pop off another key to compare.) If it's a problem with that white rubbery thing under most keys, then it's easy enough to replace with another membrane. There are some useful guides online on how to carefully disassemble and reassemble. If it's some other problem, then I think you might have to take it in for repairs.
hrm, lige while you may be right bout the lemon, the sugar makes it sticky so the key will stay in place longer. So stick the key in after the lemon-sugar-water.