If you're using Nexon Launcher to access Mabinogi, select the gear on the game title in your Library and select "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.