Is your MapleStory.exe still there? I mean is it still inside your MapleStory C drive folder?
If no, your Anti-Virus might have accidentally deleted it or have quarrentine it. If it has been quarrentined, you can make MapleStory an exception and restore MapleStory.exe back to its original location.
If your MapleStory.exe has already been deleted, you will have to re-download the entire game cilent again. Do go to your Anti-Virus and make an exception for it.