With an antenna, I'm assuming that you are looking for Over the Air broadcasts, not a cable box?
1. Check to make sure the TV is set on Air Channel reception, not Cable.
2. If using a powered antenna, make sure it's powered and working.
3. Check any and all wiring between the antenna and the TV. You'd be surprised at the amount of ppl who loose signal because of squirrels eating the wiring in attics.
It might not be you. Over the Air broadcasting is being canceled in many areas, replaced by satellite, cable and internet broadcasting.