Can you tell me why ##### members suddenly have disappeared off my mailing list

You can no longer send emails to members who have not been active within 2 years, eg not read an email, not clicked a link, not enquired, nor purchased, nor logged in... this is the same rule that many countries mandate, and is more lenient than some which only allow 1 year. If you have old members marked as "stale" then you can edit them if you want to bring any back to life, chances are they will be spam addresses or dead addresses.