This happened to mine and it had to be sent back to be repaired (3rd time) as the graphics card had failed; it started with the display colours looking strange, then the picture went totally. If you can hear everything working (beeping/whooshing" as you go through the "blades", it sounds like you have the same problem. below is how to reset to the default display sttings just in case anything has gone wrong on this.
To reset the display settings of the Xbox 360 console to the default settings, follow these steps: (Copy and pasted from Microsoft support pages)
- Remove any discs from the disc tray.
- Turn the console off.
- Turn the console on.
Note If you are using an Xbox 360 Wireless Controller, turn the console on by using the Xbox Guide button on the controller.
Note Make sure that you use the player 1 controller when you do this. The player 1 controller is the controller where the upper-left quadrant light is on.
- As the console starts, press and hold the Y button, and then pull the right trigger at the same time. This causes the Xbox Dashboard to reset the display settings to the default settings and then automatically restarts the console. So it's OK when the Xbox 360 console restarts.
If this doesn't work then try phoning XBOX Support on 0800 587 1102.