Sharepoint site down: "Operation is not valid due to the current state of the object."

When I got to the office this morning, all of our users were receiving an error when they attempted to load the Sharepoint 3.0 intranet site.  No updates or changes were made on the web server over the weekend.  This is the error they receive:

Operation is not valid due to the current state of the object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: Operation is not valid due to the current state of the object.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace:

[InvalidOperationException: Operation is not valid due to the current state of the object.]
   Microsoft.SharePoint.Administration.SPContentDatabase.ValidateDatabaseSchemaCompatibility() +689
   Microsoft.SharePoint.SPSite.PreinitializeServer(SPRequest request) +59
   Microsoft.SharePoint.SPWeb.InitializeSPRequest() +290
   Microsoft.SharePoint.SPWeb.EnsureSPRequest() +58
   Microsoft.SharePoint.SPWeb.get_Request() +54
   Microsoft.SharePoint.WebControls.SPControl.SPWebEnsureSPControl(HttpContext context) +399
   Microsoft.SharePoint.WebControls.SPControl.GetContextWeb(HttpContext context) +31
   Microsoft.SharePoint.ApplicationRuntime.SPRequestModule.PostResolveRequestCacheHandler(Object oSender, EventArgs ea) +385
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +128
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +76

Any ideas on what might have happened here?