Can be many issues. What is the model of your Dell and the version of windows you are running.
What indications do you see on the system such a the color and condition of the power button light?
Home Built Sandy Bridge P8P67 i7 2600k @ 4.5 GHz 8gigs GSkill Ripjaws. Corsair 800D Case and AX1200 Ps X-Fi Titanium Fatal1ty pro PCIe Sound Card
Dell Studio XPS 1645 Window 10 Pro
Forum member since 2003