You've almost certainly not got a problem with your TV. You've got a problem with your aerial.
Firstly, step outside and take a look at your aerial. You might find that it's pointing heavenwards, or down at the roof. If so, the cause of your problem is obvious.
Even if there's no obvious fault with the aerial, I still think that it's likely to be the cause of your problem. The signal strength at your aerial doesn't remain constant. It's affected by atmospheric pressure, weather conditions, the 11-year sunspot cycle, individual solar flares, trees coming into leaf along the signal path and many other factors. It's likely that you were getting a good enough signal until recently but conditions have changed since then. (All aerials are rather better at receiving some channels than others, because the lengths of the elements is only roughly matched to each of the incoming frequencies. When signal conditions deteriorate you'll lose the weakest channels first).
Try retuning the TV set. You might find that the missing channels reappear. However they'll probably disappear again at some time in the future if you don't improve your aerial system. (A signal booster, for under £8 from Asda, MIGHT fix the problem but a new aerial would still be a better option).
Chris