Based on my own personal expereience and that of many other posters on this forum, it is possible that the power supply unit (PSU) has failed. My Slingbox Pro exhibited exactly the same symptoms as yours and replacing the PSU fixed the problem.
Did you ever find a resolution to this? I have the exact same symptoms - when I power on the Slingbox (Pro) the network light flashes 5 times then goes off. Pressing and holding the Reset button also has no effect at all.