Do you hear any beeps when you startup the computer?
If you do, these are the error return codes from what is called a POST (Power On System Test) and they are designed for this exact problem, when you might not have a monitor to be able to display the error information.
If you do have a POST error, then if you post the exact sequence of beeps such as: long, short, short, long as this can help us diagnose if it is a video card issue.
Secondly, if you try connecting it to an external monitor do you get an image? If there is an image then we can instantly rule out video card issues and look more at the monitor itself.
I would highly doubt you've encrypted your HDD or boot loader as Jithinjohny has suggested, because it would still display a message asking for your password.