i really need help

i am now getting this message i am not a programer and i need help in fixing it. please help

Security Exception

Description: The application attempted to perform an operation not allowed by the security policy. To grant this application the required permission please contact your system administrator or change the application's trust level in the configuration file.

Exception Details: System.Security.SecurityException: That assembly does not allow partially trusted callers.

Source Error:

[No relevant source lines]

Source File: App_Web_login.aspx.cdcab7d2.3g_m2vha.0.cs Line: 0

Stack Trace:

[SecurityException: That assembly does not allow partially trusted callers.]
   PCNAdmin.WebForm1.ValidateUser(String userName, String passWord) +0
   PCNAdmin.WebForm1.cmdLogin_ServerClick(Object sender, EventArgs e) +67
   System.Web.UI.HtmlControls.HtmlInputButton.OnServerClick(EventArgs e) +111
   System.Web.UI.HtmlControls.HtmlInputButton.RaisePostBackEvent(String eventArgument) +109
   System.Web.UI.HtmlControls.HtmlInputButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +10
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +13
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +36
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +6785
   System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +242
   System.Web.UI.Page.ProcessRequest() +80
   System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context) +21
   System.Web.UI.Page.ProcessRequest(HttpContext context) +49
   ASP.login_aspx.ProcessRequest(HttpContext context) in App_Web_login.aspx.cdcab7d2.3g_m2vha.0.cs:0
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75