If it is a Kindle1 - There is a reset button located underneath the cover on the back of the unit.
At the "seam" on the back take your thumbs and press firmly down while sliding the cover down.
Look for a small hole above the battery compartment. Use the end of a paperclip and insert into the hole to reset the device.
If it is a Kindle 2 - just press and hold the power button for 30 seconds to reset.