This may be a hardware issue, which may require a visit to your local Apple Store.
But try this, first. Power the unit off by pressing the on/off switch on the upper right corner. Then, press the Home button and on/off switch at the same time, for three or four seconds. Finally, press that on/off switch in the corner once again. Normally, this will reset the machine - unless there's a hardware issue.
Post back with the results!