Apart from all the possible causes Fernandez gives even the weather can change reception. Until I upgraded my ariel I could lose several channels during the summer. Sometimes a booster can help, but reception depends on quality as well as level. If you do nothing you might even find that the picture quality returns as the weather changes. However, that's likely to be temporary.
If you enter your postcode here (
www.wolfbane.com/cgi-bin/tvd.exe? ) it gives ariel advice in the far right hand column. You may find you need to upgrade the ariel apart from any other actions.
Hope this helps.