Contrary to all advice (about using lubricant in lock barrels) I have fixed three cars with that problem by injecting WD40 (My excuse is that I reckon that it isn't a lubricant.) into the lock barrel and then working the key in and out for about five minutes. Don't try to turn it just move it in and out. The way that I see it you have nothing to lose by trying this.
t, January 2008