If you are not seeing video on an external monitor or through your camera, you should first make sure everything is connected properly. If the below steps don't work you should try to quit FCP, turn your camera on (make sure it is in tape mode) and then fire up FCP.

Go to View, and down to Refresh A/V Devices. Wait a few seconds to see if the video "wakes up."

Then go to View and External Video. Make sure it is set to All Frames.

Then check to make sure that FCP can see the FireWire connection by checking View and Video Playback. Make sure the above is selected. You may need to reset the External Video setting after making the changes.

And since we are here, you may want to make sure audio follows video by selecting the Audio Follows Video setting in the Audio Playback under the View menu.