Enable Logging from the Troubleshooting section of the GSX settings and, if it happens again, exit the sim, check the %APPDATA%\Virtuali\Couatl.LOG file, ZIP it and Attach it to a post here.
It's important you exit the sim as soon as you don't see the GSX menu anymore. If you restart the sim, the relevant log would be lost and a new one will be created.