If after upgrading to the the latest ETS version, ETS is shown in the task bar but you cannot use it
then press ALT+TAB, keep ALT pressed and select ETS.
If this still does not solve the problem, 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 'C:\Users\YourPcUsername\AppData\Local\KNX\ETS5' 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 ETS5 again.