Hi lustigs, are you able to connect using MySQL Workbench and the root account? If you previously connected to MySQL Workbench and checked the "Save password in vault" box on the connection window, then you might be able to connect again without having to remember the password.
If you're able to connect to MySQL Workbench, you can reset your password on the "Users and Privileges" page. After resetting the password, open the Sharing Options in VueMinder and enter the new password.
If you can't reset the password, the only other option would be to reinstall MySQL Server and re-configure VueMinder to share calendars. You can uninstall MySQL Server via the "Uninstall a program" area of the Control Panel. Make sure to exit VueMinder before uninstalling MySQL Server. After uninstalling, you can re-install by following the instructions linked below:
http://www.vueminder.com/sharing.php