Hvae you retuned your TV? You may now be recieveing the channels from a different transmitter on different frequencies. Or the coax/ aerial may be faulty, or you may need a masthead amplifier which amplifies a weak signal.
If there is a masthead amp already fitted, it will usually be on the pole holding the aerial and be a smallish square/rectangular plastic box.
You will also need a power supply for the masthead amp to work, which looks a bit like the mains power supplies for kids games etc but it has a place to plug in the aerial lead. Any good aerial rigger/TV shop should be able to supply and/or fit whatever you need.