If you have previously had good coverage, and you have now lost some of your channels, try retuning your set top box to reload channels. If channels are still missing you may need to clear the memory of your box before retuning. Make sure it is turned off and unplug the aerial. Turn on the equipment again and retune without the aerial; your box won't find any channels. Turn off the equipment again and plug the aerial back in. After restoring the power, do a full retune using the first time installation option (sometimes called default or factory settings). You should find your channels return. If you are not certain about how to do a full retune of your equipment, check your manual for instructions. There are also local factors to consider if you have lost a block of channels only, for example a portion of the local signal may have been reduced in power or work could be being carried out.