I just got a new HDTV and I plugged everything in and I am using the HDMI cable, but I am not getting a signal. I also have the coaxial cable plugged in and can't get a signal from that, either.

The TV is a Samsung LN52A550, and the cable box is a Motorola DCT3416 I. Any help would be greatly appreciated!

Are you able to select the HDMI connector you connected to using the source selection button on your TV remote? If yes is this when you get a no signal indication?
Have you previously used the 3416 with another TV or display?
If you connect your cable directly to the TV instead of to the 3416 can you receive the analog channels?

Is there anything on the display; a clock or input display or something?

Do you have component/composite/s-video cables you can try instead of HDMI?

Many HDMI inputs on newer TV's will not accept 480i video input. Check the video output setting of the 3416 and set the HDMI to 1080i.

Note... to do so, you may need to connect the 3416 temporarily to the TV using composite, s-video or component cables.