Is everything plugged into the correct sockets, e.g. scart lead, and if they are then are they securely plugged in, like pushed right home into the plug/socket?
Sound and vision are on different pins of the scart plugs - very easy to have a connection on one and not the other. Try taking the scarts out & check that none of the small flat pins are bent over.
Whoever designed these connectors (or decided to use them in domestic situations) wants shooting - they are just so prone to being 'not quite inserted properly'.
As others have said - an improperly insertes SCART lead can still function to an extent. In fact there are often questions on here about missing colours, sound, etc. which are almost invariably resolved by checking that the plug is fully inserted.
Does it display anything at all (even 'white noise') when you select a channel using the TV's own tuner - presumably an analogue one that won't pick up any channels now?