I am having trouble booting up. The screen stays black for up to 30 minutes and then a folder with a question mark shows...

Thank you for helping. I took it to Apple Store today and it wouldn't do it again, however he couldn't get the video to come on the display. He wanted to do a diagnostic tests. we feel at this point the logic video is out. I think the HD is still good. Do you have any ideas how I should retrieve the data?