1. If you are logging in via the GameLauncher.exe found in your MapleStory folder, try logging in from the site instead. If you're logging in from the site, try the GameLauncher.
2. If you're logging in via your Nexon ID's username, try to login via the email address associated with the account instead. If you're using an email address, try logging in with your username (if you have one).
Shitty Nexon, driving players to hacking forums to get help for problems that should be solved through their ticketing system. Let me know if you get it working, good luck.