how do I fix my computer if i only get a black screen on startup

That means you take it in for a diagnostic from a professional. Take it to your local Apple Authorized Service Provider, if you don't know of any click Apple Authorized Service Provider Locator.

I know that is not what you wanted to hear, however it's what you need to do if you want to fix the computer.

Daniel R from Luck wrote:

When I start up my computer I get a black screen, how do I fix this. I pressed the D key for diagnostics and go no response.

Did you change your video card? If the card does not have Mac firmware on it -- even if it is the correct card you will get a black screen unfortunately.

Other than that go to the repair shop (didn't like the Imac option because of that attached screen)