You can almost always take the Weak or No Video Signal Detected error at it's word- the Slingbox is not being fed a signal.
I suggest you check your video source (cable/sat box, DVR, etc.). Make sure it's turned on, hooked up and working properly.
Another thing you can check is if SlingPlayer is set to the proper video input. You'll find that in your menus, the Sling AV has two input choices: Composite and S-Video. If the wrong one is selected, you would likely get a Weak or No Video error.
Cable boxes need to be rebooted once in awhile, sometimes they go goofy. If that's what you have connected, try disconnecting it from power for 30 seconds or so then plug it back in. Turning it off with the power button usually doesn't turn it off. It just puts it into low-power standby while the guts are still powered up.
Finally, try connecting a different video source to your Sling AV if possible as a test.
So you know, that message only appears if you have connected successfully things are working right. It's the Slingbox equivalent of static on an older TV.
Specifically, it means that you have a successful, stable connection to the Slingbox, it accepted your hardware password, it has loaded the source profile and remote control, it's sending you a stream but it it finding no signal on the selected input to encode. Check the lower right corner of the window, it should be registering a live stream at very low bitrates.
I'm not saying your problem is as simple as "just turn it on" , but you definitely want to check your video source. Here's an example of a healthy, fully functional Slingbox AV. Note the bitrate/buffer meter in the lower left corner...
A screenshot of my friend's Sling AV. I'm connecting over the internet (2200 miles away), and the video source (a DVR) is turned off:
(Click to Enlarge)
Here's the same Sling AV after I click the power button on the on-screen remote:
(Click to Enlarge)
Hope this helps,