It's possibly a defective digital tuner, but the vanishing menu suggests a more serious (and expensive) fault with the main/AV board, assuming it's an LCD TV. It's impossible to be certain which one without doing some testing, but due to the inherent dangers with high-voltage electronics, I don't advise on self-repair for TVs.
If you're looking for an inexpensive solution and the TV is no longer covered by any warranty, I'd suggest buying an external Freeview box to connect to your TV. This would also enable you to upgrade to Freeview HD. Being 4 years old, your TV will not have been fitted with a DVB-T2 (Freeview HD) tuner.
If you want to try this solution, I can recommend the following box, which is great value at just £25.
They are described as "reconditioned", but I've bought and installed 2 of these for my own customers recently and both have been brand-new. The majority of these are unused returns which Tesco are consequently not allowed to sell described as "new". For a lesser known brand, I've been quite impressed with some of the recent Technika products and they are easy to install and come supplied with all the necessary cables, etc. Additionally, if it doesn't work for any reason, you will be able to return it under UK Distance Selling regulations.
Before purchasing this (or any other) Freeview HD box, please check your TV has a spare HDMI input socket as SCART cannot carry an HD signal. If you don't have a spare HDMI socket, I'd also check your TV has a spare SCART input socket or you will need to disconnect your Sky box every time you want to use the Freeview box - which brings me to my final point . . .
. . . Off the top of my head, I can't think of any Freeviews channels which aren't available on Sky, so as long as you can still use your TV for Sky, do you really
need to spend any money on it?
Hope this helps. Please let me know if I can be of further assistance.