All of the missing channels use the same frequency. (They're part of the 'PSB2' multiplex).
The laws of physics dictate that your aerial will be better at 'pulling in' some frequencies than others and I suspect that you're aerial doesn't resonate well with the frequency used by PSB2.
So, when reception conditions weren't at their best (possibly because your aerial was being battered by Storm Ciara) there wasn't a strong enough signal for your Humax box to be able to 'lock onto' the PSB2 frequency and the box dropped all of the relevant channels from those that it could work with.
Turning it off and then on again forced it to try again to 'lock onto' the frequency which, because reception conditions had improved, it was then able to do.
If the problem keeps happening, install a signal booster into the aerial cable to make your Humax box's task easier for it.