ASUS Computer Kerfuffle - System failed to boot up.

Any ideas? I have everything plugged in and everything works fine, but when I come to turn it on, the monitor remains blank saying 'Check VGA Connection' and the computer on and the speaker blaring 'System failed to boot up' I think. It sounded more like 'System failed to bluetack' but I am confident in saying that this is not what it says. I've tried replacing the graphics card and all the connections but I have had no luck. Please, if you have any ideas, post them in the comments.