Some customers have run into an issue that their RIA Services app used to work fine, but for some reason the app based on JSON or Soap endpoint no longer works and the domain service wizard is not displaying LinqToSql data context anymore.
If you've seen the same thing, the reason might be that you've recently uninstalled and reinstalled WCF RIA Services V1 for Visual Studio 2010.
We have a setup bug for V1 that the WCF RIA Toolkit registry keys are deleted along with the uninstall of the product (WCF RIA Services V1 for Visual Studio 2010). Reinstall of the product itself is not going to put the registry keys back. As a result, the toolkit functionalities which include the JSON and Soap endpoints and support for LinqToSql by the wizard are gone due to the absences of the toolkit registry keys.
A temporary workaround is to uninstall the WCF RIA Services Toolkit from your machine and then install it back.
Our team has fixed this setup bug for our next release. Sorry for the inconveniences and stay tuned with WCF RIA Services!