NullReferenceException was unhandled by user code

Question

 I got "NullReferenceException was  unhandled by user code : Object reference not set to an instance of an object" on selecting or clicking any control after saving same webpage once. First time, I am able to save webpage  or complete transaction successfully after making selection in various drop downs and entering values in text boxes. After saving, as soon as I click on any of the control on page, I got this error. Other pages of website work properly. Any help will be appreciated.