1. Make sure the input on the TV matches the one that the laptop is plugged in to
2. Many notebooks (almost all) have a function>F4 or some key combination that triggers the computer to enable the secondary monitor. It's different depending on manufacturer. My HP is Fn>F4 I think Dell is Fn>F8 It will show a little picture of a screen on the F-key that is used.
3. If those don't work are your cables tested working? Make sure the connection is secure.
4. If these still don't work it would be helpful to know how you are connecting the two AKA HDMI/S-Video/VGA