Power it down, and open the hdd bay, remove the hdd and then make sure all the connectors are clean you a dry tissue, then plug in the hdd and close the bay up. If that doesn't help the hhd is likely dead and needs to be replace, if the laptop is still under warrenty contact dell support, they might be able to help.
Couldn't have said it better than myself. What KnightSword said.
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