First, try tuning the box again. If there's an choice of options between 'add new channels' and 'replace existing channels', select the latter.
That might fix the problem but, even so, it's likely that it reappear. Missing channels (with an otherwise fully functioning digibox) are invariably a symptom of insufficient signal strength. Each channel is on a slightly different frequency. Theoretically you need a separate TV aerial for every single channel which you watch (because the aerial elements should ideally be exactly the right length to 'resonate' with the relevant frequency). Obviously that's not possible, so any aerial can only offer a 'compromise solution', with the lengths of the aerial's elements being better matched to some channels than to others.
The strengths of the signals received at your aerial don't remain constant. They're affected by path obstructions (such as a tree growing in front of your aerial), sunspots and solar flares on the surface of the sun, the time of day and the seasons of the year (because the reflective layers of the earth's ionosphere move up and down depending on the amount of sunlight) and lots of other factors.
Additionally, the effectiveness of aerial installations can deteriorate over time because of things like water ingress into the connections or the aerial being knocked out of alignment by storms.
So you need to think about increasing the strength of the incoming signal. Take a look at your aerial and check that it's still pointing in the same direction as others in your street. If so, try using a cheap signal amplifier. If that doesn't work, it's probably time to get a better aerial.
Chris