When TV channels
are moved from one frequency to another frequency that was previously used by a different channel (sorry, but there's no simple way of saying that !) as has happened recently in th UK, some digital TVs
and digiboxes struggle to sort the channel list out properly and channels can go missing as you describe.
If your equipment has the option to delete all currently stored channels (normally in the tuning menu options) then do so and then do a full re-tune.
Sometimes, this solves the problem as the equipment is no longer aware of any previous use of a frequency and just loads every channel it finds.
If this doesnt work, or your equipment doesnt have the option, then there is normally the option soemwhere in the menus to retore the item to "factory defaults/configuration". This will clear all previously stored channels and after a full re-tune your missing chanels should be there. However, any other configuration changes you previously made, e.g picture
size, output type (RGB/NSCT/etc), etc will be lost and you'll need to set these again.
NB. some regions of the country have lost some channels temporarily now until after the switch over programme completes at the end of 2012. If you visit the Freeview website, this will tell you if your area is affected.