At a guess I'd say it is down to resources on your PC? Close Producer, kill any applications you don't need, that may be consuming an inordinate amount of resources and try again. If that doesn't work, reboot (sorry, I know that's the Help Desk catch-all, but...).
If it is not that, make sure all your Resources are up-to-date (re-synchronize).
Is it happening for all simulations, and/or for all users? If so, check if there have been any changes to your browser and/or network settings. Producer actually uses the browser to display everything, and it could be that something is now blocking it.
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