MacBook Pro - start up problem

I have had a problem with my airport. When ever I tried to connect to a wireless network, typed in the password, the notebook started processing it but did not stop. I had to force shut down the notebook, as I did not know how else to stop it. How can one close an application when it is not responding to anything? anyway, When I restarted it I was concerned.

When the screen lit up, a picture of a blinking picture of a folder with a qustion mark appeared instead of the Apple picture. It kept blikning for a long time. The computer eventurally boots up but it takes longer and the picture of the question mark appears. What is this?

thank you for your help.