As channel numbers 1 to 299 (and also 600 to 799) on a BT TV box are Freeview channels, whereas channel numbers 300 to 599 are internet-based services, it appears that one of two things has happened. Either
(a) there's a fault with your aerial (such as it having fallen off your roof, the cable having been severed, a signal amplifier in the lead having packed up or, simplest of all, the aerial lead not being plugged into the back of the box) ; or
(b) the TV tuner circuit inside the box has packed up.
Either of those would leave you with internet-based services but no Freeview channels.
So, firstly, thoroughly check the everything is OK with your TV aerial and its cabling (and that the plug is securely pushed into the socket on the back of the box). Then, if that doesn't work, explain the problem to BT and hassle them for a new box.