my 2001 toyota MR2 won't start,won't even make a sound,just the click of the key and only starts when ever

won't even make a sound,just the click of the key and only starts when ever,some times
i'll drive it for days with out any problems but then  i'll go to the market and just won't
start,some times i have to leve the car seat for hours or even days so that it will start
again..do you know the answer??