imac will not start black screen with white arrow operated by mouse appears after slow boot up. I have tried holding down shift key which results in prompt to enter name and password but black screen appears after this.

When I try to start my imac(8 years old) after the screen showing the apple and bar with booting up progress showing instead of the desktop display I just get a black screen with a white arrow that can be moved with the mouse. I have tried pressing the shift key when I switch it on which brings up a request for apple id and password and "safe boot" in red in the top left hand corner but again I end up with a black screen after some flipping of lines and amixed colour background. I fear it might be beyond fixing but have a lot of files I need on it.