Click on your Nexon Launcher profile name in the top right and click "Launcher Settings." Then, select "Game Configure" and choose Mabinogi. Then, click on 'Repair Installation'
If you're using Steam, right-click the game in your Library and select Properties. Under the "Local Files" tab, select "Verify Integrity of Game Cache..."
If you are using the original client, you may be required to redownload and reinstall the game via the website.
The game installation is under the "Clients and Drivers" page in the "Support" tab.