If you are using component cables (red green blue) make sure you inserted the correct color into the correct corresponding colored jack.
If you are using a Wii2hdmi converter, maybe it's faulty. If you aren't using one maybe it's time to get one, they are fairly cheap.
If you are using composite cables, try using a different known working cable.
In my experience this "usually" happens because the person is using component cables and plugged the wrong color plug into the wrong jack. Good luck!