If ETS is shown in the taskbar, but you cannot use it, then:
1. Ensure that ETS is not running at all. End the application from the Task manager, i.e. ALT+CTRL+DEL-->Task Manager.
2. Browse to folder '%LOCALAPPDATA%\KNX\ETS6' and rename the file 'user.settings' to 'user_old.settings'.
In case you cannot find this folder, it may be because it is hidden. See here to read how to make it visible.
3. Then start ETS again.