That depends on a number of factors. Please check the MapleStory website to see if the game is undergoing maintenance.
If the game is not undergoing maintenance and you're getting a specific error message, you can search the message in this database to find the cause and solution.
You can also find help on the Technical Support section of the Official MapleStory Community forums.
Otherwise, please submit a ticket with specific details about the issue by clicking on the button at the top of the page.
The more information you are able to provide, the faster we'll be able to assess the problem and provide a solution.