One of the most frequent reasons is that you go away for a few weeks and your mailbox fills up, causing messages to be returned to sender. If enough of these bounces occur, we assume your address is no longer valid and the listproc automatically removes you from the list. So if you don't hear from the list in a few days, try re-subbing to the list. You will either get a confirmation that you are now subscribed or an error message saying you are already subscribed to the list. Usually most lists have a digest form.
Some lists have archives.
You can send a review command to the listproc to see if there are any archives for that list. If archives are created for this list you can send an index command to get a listing of all archive files available. The review command is used to view the list of subscribers for a list as well as other attributes of the list.