A fair number of people are having issues with the ASP.NET control used to host Silverlight applications ceasing to work when they upgrade from Silverlight Beta 2 to Silverlight 2 RTW. If the HTML page is also broken, the first thing to check is that the HTML markup is correct, since the MIME types have changed between the two releases. In RTW, the HTML <object> snippet now looks like this:
<object data="data:application/x-silverlight-2," type="application/x-silverlight-2" width="100%" height="100%">
Note that both the data type and the application type have end with -2. The reason for changing the data type was to resolve an issue on Firefox. That scenario was that Firefox with Silverlight 1 (but not Silverlight 2) would try to run the application with the wrong plug-in; it was not an obvious issue when debugging, so do double check both types.
If the HTML page is still not working, then it's possible that the Silverlight application is broken. Review the breaking changes (or see Mike Snow's post on it here) to make sure that your code is correct.
Once the HTML page is working, if the ASPX page is still not working, the issue is most likely caused because the System.Web.Silverlight.dll on the server is out of date. The assembly from Beta 2 would output the incorrect MIME types; if you purge it from the server and replace it with the version in the RTW SDK (2.0.30825.0), it should begin to work correctly.