It seems that you are having some issues with your Slingbox PRO-HD, since the power light is out.
I recommend you to reset the Slingbox. The following link will provide you with the steps to follow:
If the issue persists, it issue might be related to a defective power adapter. If this is your case, I encourage you to check this announcement:
Hope this helps!
While your power adapter may be generating the necessary 5 volts, it might not be able to deliver the necessary 4 amperes. So, I would first try replacing the power adapter. If your unit is still under warranty, then Slingbox folks will replace it for free. If your SB is no longer under warranty, then you can purchase a replacement from them or from a third-party supplier. I purchased a replacement for my older SB-HD from bgmicro.com. So far, the replacement is working fine.