Just started doing this tonight.  Can't get past the welcome screen. Waited 15 minutes a couple of times before giving up. X2 starts normally.  Uninstalled, reinstalled, no help.  Ideas?

Are you trying to open a project or just launching SONAR without a project when it hangs?

Never gets to opening a project.  Hangs at the welcome screen.

Did you remove the registry entries after removing SONAR and before reinstalling? If not, you should consider another attempt at reinstall. Here are the instruction for a clean reinstall of X2 Producer, the process would be the same for X3 Producer except for the program name change.

Have you tried to bring up SONAR with the SHIFT key pressed when you click on the program icon? That will force a personalization of sonar. Maybe something is out of sync.
 
Also, do you know if you have scan VST plugins on each start?
 
I have seen SONAR keep dialogs in the background and it will appear as if SONAR is hung. I don't know if that is the case here or not.

I'm not sure why, but it just started working again.  I'm not reassured. Kontakt hung on a patch that had been deleted from a project during startup. Other weird intermittent stuff. I'll keep an eye on it, and if things go nuts again I'll implement the (much appreciated) suggestions.