I don't no why it has been changed suddenly , i didn't make any changes. Can you say me what is the problem?
This can be because, either this has become a numbered list and numbered (Either in the general list or in the configure) has been checked or the display name has been erased against this numbered list if it was already a numbered list
The 'Numbered' checkbox is probably checked for 'Sales Rep L1'.
Go to 'History' and look for 'Description = Change List' to 'Sales Rep L1', and have a discussion with the person who did it.
Then i'm guessing someone turned it to a 'numbered list' (changed names to #xxx), and then unticked it. (#xxx remains).
Once again, history is your friend.