This might sound like a silly suggestion, but have you restarted your computer? That sometimes does the trick.
But if it doesn't, and you've been using the Gameforge Launcher (GFL) to install and start your game...well, it would be far better to get rid of it. That will require opening Control Panel, running the Programs and Features applet, and removing both RoM and the GFL.
Once you've done that, go here:
Manual Downloads for the 6.0.1 RoM Client
Read what Inferiority says and follow his directions. The ten (10) installation files are clean and virus-free. Get them downloaded and installed, then run the client to get it patched up.
Hope this helps.