If it needs reset as you say then you can do that yourself by pulling the power cord for a few seconds.
It would be better though, if you said why you cannot get those channels, is there an error message? Not authorized?
Do what John suggested of power cycling the box by unplugging for 20 seconds and plug back in. If the issue is still there after that, and it's only one box with the issue, then you should replace the box either by trading it at a Cable Store or having a tech visit.
I had this issue as well.
They will run a scan.
Everything will be working fine on their end.
They will reset the box.
That won't work.
They will repeat the above.
Still won't work.
They will schedule a home visit and replace the box.
Called four times before I could get a tech to do as I asked.
If you have different boxes in your home, have the tech copy the code from one of the boxes that currently works.
Have them paste the code in the box that isn't working.
The non working box probably has an incorrect code (non HD code)
After two hours, and on my fourth call, the tech finally did that. (took him 30 seconds)
Presto...the box works.