Not sure I'm afraid - in the past we have culled users who haven't logged in for a very long time, e.g. 6 months or a year. We've stopped this now as it does occasionally cause these issues.
If that has happened then it's unlikely your old posts have actually been deleted but will have been set back to "guest" status.
Can you link to any of your old posts? It's possible we can link them back to your current profile.