Error Type: Exception has been thrown by the target of an invocation.

I have created one chart web part and configured with excel service and successfully displayed data on webpart.

After sometime i reopened my SharePoint site and then suddenly am getting the following error.

 Exception has been thrown by the target of an invocation.

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Microsoft.Office.Excel.Server.WebServices.ExcelServerApiException: An error has occurred. at Microsoft.Office.Excel.Server.WebServices.ApiShared.ExecuteServerSessionMethod(Boolean hasSessionId, String sessionId, CoreServerSessionMethod coreWebMethod, String name) at Microsoft.Office.Excel.Server.WebServices.ExcelService.GetRangeA1(String sessionId, String sheetName, String rangeName, Boolean formatted, Status[]& status) --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeTypeHandle typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.RuntimeType.InvokeMember(String name, BindingFlags bindingFlags, Binder binder, Object target, Object[] providedArgs, ParameterModifier[] modifiers, CultureInfo culture, String[] namedParams) at Microsoft.Office.Server.Internal.Charting.Data.ExcelWebService.GetRangeA1(String sessionId, String sheetName, String rangeName, Boolean formatted) at Microsoft.Office.Server.Internal.Charting.Data.DataSourceExcelService.get_Data() at Microsoft.Office.Server.WebControls.ChartDataBinding.GetEnumerableData(Chart designtimeChart, Int32 index) at Microsoft.Office.Server.WebControls.ChartDataBinding.Bind(Chart designtimeChart, Chart renderChart, Int32 index) at Microsoft.Office.Server.WebControls.ChartWebPart.BindData(Chart designtimeChart, Chart renderChart) at Microsoft.Office.Server.WebControls.ChartWebPart.PrepareRenderChart(Chart designtimeChart, Chart renderChart, Boolean initialize)

And also am unable to reconfigure through Data & Appearance....

Go into the server running the excel services.

and reset IIS.

Then my problem got resolved. But actual reason for this is do not know. if anyone knows let me know about it pls.