Error in Redirecting Users to another page after selecting drop-down Role and inserting password .

I have a page which has a dropdown called Role and also a textbox called password, now the idea is this, when a user selects role and inserts password in textbox password if the condition is met the user will be redirected to page A or else display error message to the user " you selected a wrong role".
I have tried with some codes but am getting error message.
The application will fatch the Role and password from the users table.

Also the application will check if the user is autenticated and if the user is authenticated then the application should check if the role and password selected by the user is correct.

Please Note: this is not my login page, this is page that will check and redirect user to a proper page based on the role and password.

Here is my code behind:

protected void Page_Load(object sender, EventArgs e) { if (!IsPostBack) { ...