If your TV has lost its channels or even just one channel, the TV will usually display a message on the screen, eg "NO SIGNAL" etc, this means the TV is no longer receiving enough or any signal at all.
In some cases this is due to a fault in the TV itself, the TV tuner could be faulty or the tuning has changed and the TV only needs a channel re-tune.
In a lot of cases this is due to antenna problems or cable problems in the room or in the roof space, ducts etc..
The process to find the source of the reception issue is often quite easy, if the antenna has visual signs of decay.
Iin other cases it will require time to investigate, this can become quite complex in cases where the antenna system has been worked on over the years with out adhering to the guidelines of antenna technology