Message Box

Hi there! I try to use Message box using C# in asp.net. I did find some sources on the web but it gives error and doesnot work.let us say that I have a log in page and if the user type incorrect details, a small window will pop up to say "sorry try again" or sometrhing like that. Can you tell me how to use the message box or window to pop up providing the code. I appreciate your time and help. Thanks for reading the message. Kind Regards Fadil Alnassar