Make sure you perform the factory reset as prescribed in this article (http://support.slingmedia.com/get/KB-005036.html). Check for the blinking lights as listed in the article.
If they don't blink appropriately, then check the power supply. Make sure you're using the appropriate one. On the bottom of the Slingbox there's a label that indicates the appropriate power configuration: voltage, amp, and polarity.
Make sure that this matches the power configuration listed on the AC adapter. If that still doesn't work you can try a compatible one, but make sure that it matches otherwise you can burn out your Slingbox. And if that doesn't work, then most likely you'll need a replacement.