The TV is only showing half of the picture
There is not much for me to go on with your description but here goes,
I suppose you have a convential TV set with a glass picture tube and not one of the new types with the large screen slim style displays!!!
If it a glass type picture tube then the following assumptions are made.
I hope yopu have not shifted house recently and moved the TV around???
If you have its possible the Yoke (its a device on the neck of the Picture tube that controls magnetically the scanning dot) may have moved and only is letting you see half a picture...
This is a long shot but it could be the cause..
Now the picture is supplied to you by a fast moving dot of light (Black & White set) that scans from the top of the picture and moves very fast down to the bottom of the screen, then flys back to the top of the screen and starts all over again during this process the dot lights up the picture tube to provide you with
the picture you see... (On a colour set there are three dots)
Its quite a complex process and its all controlled by electronics inside the TV receiver.
If you are getting only half a picture then something is causing the dot to only turn on for the lower half of the picture during its write time to the glass screen,
You dont say if the half thats off is white or black, so there are many things that the service technician needs to know before even starting to look for this fault. During the dot doing its write cycle down the sceen, when it reachs the bottom and flys back to the top of the screen it receives a synchronous pulse from the TV station, and this TV signal is critical in making sure you see the same picture as is being sent from the TV station.
Sometimes this sync pulse from the TV station is very weak and so your picture may float up and down the screen trying to lock to the incoming signal.. this just may be your fault.. but you dont say if the half of the picture that you see is floating or not..
Your situation may be that your TV has partially locked to the station signal and your aerial may be problematic,
but then you dont say if its on just the one channel or all of your station channels. So there needs to be more analysis of your situation prior to a positive answer being provided....
Sorry I can be of more help...