There are a few steps you can take advantage of to troubleshoot this issue.
This is applicable to the Nexon Launcher only.
The first is to uninstall the game fully and then re-download the game using the Nexon Launcher. Be sure after you run the normal uninstall that all files related to the game as well as any folders are deleted from your system.
If you already use the Nexon Launcher you can run the "Repair Install" feature for the game to check for any issues.
You can also rename or delete the file labeled; “CurrentInfo.txt” doing this will prompt the game to patch. Complete the patch and then try running the game again.