Lost channels on freeview?

I have a venturer TV and DVD player built in and the other day loads of channels had no signal so i retuned the channel and hardly have any channels left. Just main 4 and BBC ones. does anyone know how i can get all my freeview channels back? digital switch over isnt until 6th july for me, is that something to do with it? i figured it wasnt the aerial as surely would have any channeld if it was?