If after upgrading to the latest ETS version (>=5.7.0), your ETS does not start, then it could be that the 'master data' file is missing/or it is corrupt.
How to identify?
If you open with the notepad the 'ETS.log' from the directory 'C:\Users\YourUserName\AppData\Local\KNX\ETS5\Log' and you find an error similar to that one:
How to solve?
Go to https://update.knx.org/data/XML/project-20/knx_master.xml and download this file to 'C:\ProgramData\Knx\XML\project-20'.
In case you cannot find this folder, it may be because it is hidden. See here to read how to make it visible.
Then, start ETS again.