Prevent or rollback the postback on page_load

[DIETTURNE]

First of all, I can't use Async plugins like Jquery, Ajax, UpdatePanel, etc.I have an ASP.NET 4 WEBFORM, In the master page I verify if the Session is lost, in that case a show a modal to get the username/password and log-in the user again.But I've two problems, the first one is that for example if I press a button and I lost the session, the webpage show me the modal but the code behind of the clickevent still execute it and i want to prevent it (I dont want to verify the state of the session in every event handler of the APP).The second one is that after Im success on log-in the user, i want to finish or continue the event that the user tries before he lost the session.I other words my problem is that I need to prevent the postback in the middle of it, and then do it again. :/Something like this:\[code\]Page_Load(...){If(Session["User"] == null){ PostBack.Cancel(); //Like de e.PreventDefault() of Javascript. return;}..}ANDLoginOnSessionLost(){...//After Login SuccessPostback.Continue();}\[/code\]Any suggestions? Thanks