If you hit it against something, it is possible that a couple of things might have happened.
1.) The screen is damaged internally and will require a replacement
2.) The video ribbon cable came loose and need to be reseated. You will have to take the unit apart.
In either of these,,, if the system is still under warranty, you are best off by contacting Dell and have it repaired.
To further test... I would hook up to an external monitor and see if it will boot, If you are having the same issue, it could be other things.
If you have no problems booting to external monitor, then it is an issue with the LCD, video cables or Video Card