Theres a harder reboot method that is used to clear all the cache from the boxes memory. You may need a friend to help - Press and hold the power button while the box is on, then pull the power plug (this is where a friend may help) but continue to hold the power button. Wait about 8-10 seconds - all the while holding the power button - and re-plug in the box to the power outlet. You may see some codes flash on the screen, and you may see the typical hex countdown, start with rxxx.
This is used to fully clear out the memory, and reload the o/s. The box should pull all the data from the network and may take upwards of 15-20 minutes to load.
Really the only servicable part in these boxes are probably the hard drive - if you have no luck with the above instructions, you may want to remove the hard drive, and try to boot without it, or track down another sata drive (one you KNOW is working) and swap it out and try to reboot to see if (original) the hard drive is dead.