What make and model of TV do you have? What is your signal source (OTA (indoor or outdoor antenna), cable, satellite)? How is the TV connected to the source?
The model is important since some "TVs" are actually only monitors and do not have tuners. Then you need to connect to a set top box to watch anything.
Assuming the TV has an ATSC (digital) tuner for OTA, check that your antenna is pointed in the correct direction for the stations in your area. Use antennaweb.org
to identify the direction. Antennaweb assumes an external antenna and gives a very conservative list of stations. TVfool tells you the type of antenna needed to get any station (assuming no trees or tall buildings that might block the signal). Depending on your TV, you will need to enter the menu in order to correctly set the incoming signal type and scan for channels. Digital signals are either enough for the tuner to interpret or not available. If your signal is split between several TVs you may need an amplifier to get enough signal. Sometimes, the amplifier may fail and filter out all stations in certain frequencies. (Older TVs often only had a NTSC tuner; there are very few NTSC stations still broadcasting. They are low-power and thus you may not be able to watch them if they are far away.)
If you have cable without a box, which type of cable signal is coming in? Digital cable is QAM and not all TVs have a clearQAM tuner (that reads the unencrypted signal).
If you are using a set top box, make sure to set the TV to the correct input (HDMI, component video, composite video, coax and the correct number if there are multiple inputs). Make sure that the box is also on. If the set top box requires the TV to be set to a specific channel (3,4,60), make sure to go to that channel.
I hope this helps.
(For the US, the digital OTA tuner is ATSC and QAM for cable. In other countries, different signal formats are used.)