When you have lost ALL channels, chances are you need new keys, and/or autoroll is not working.
When you have lost SOME, but NOT all channels, delete your channel list and do blind scans to fix. Quite often channels are being moved from one TP (or satellite) to another, which causes this.
In a less likely case, an ECM could be causing loss of SOME, MANY, or ALL channels. In this case, you must wait for a new bin. Once again read messages on forum, the gurus will let you know when this has happened.
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Hint: you will know when keys have changed (and your autoroll is not working/on) because the forum will get a LOT of traffic for "loss of all channels for many people". If you are not sure, get keymaster program and compare keys, displayed by this program, with your receiver's keys.