You have to programme the correct code for your TV into the Sky remote.
You can get these via "Sky Active" (if your box is connected to a phone line) or from the Sky website, or google search: "Sky remote codes".
However your sky remote wont necessarily control more recent televisions unless you have a REV8 remote control. You can check this by taking the battery cover off your remote and looking at the sticker inside. If it says RC1600/00 you're probably not going to be able to control your TV. REV4 and REV6 are more likely to work - but not guaranteed. If it says REV8, then you are probably not using the correct code or you're not programming the remote correctly.
If you have REV8, the correct code for your TV (according to Sky) is: 0678 or 1678 or 0254
Sky remote code finder page is here: http://sky.uebv.com/
Sky guide to finding your remote code and programming remotes is here:
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