Yellow means that you are in suspend mode or in hybernation mode.
Use the usual procedure to exit those modes. Tap a few keys, move the mouse, quickly press the power button...
If that don't work, try pressing the reset button to force a complete restart.
If that still don't work, unplug it completely and wait a few minutes. If it's a laptop model, you need to remove the batry. DELL XPS' commes in both desktop and laptop configurations and you don't mention the version you have.
Plug it back in and power up.
Electro, September 2011
