Greetings, and welcome to the forum!
When you download data to the computer,, do you get good video? Assuming this is a Solo (not that it matters much)?
Anyhow, the GoPro3 defaults in the "record video" mode whenever you manually turn the camera on. You should see the blinking red light at preflight.
If no blinking indicator, check the battery and your camera's settings. The cam's default settings may have inadvertently gotten changed, or scrambled. But you can always revert back to default settings.
However, If it records video, but doesn't downlink to the monitor, it's either the transmitter, monitor, or a bad cable going to it.