This can be caused by logging off the client and then shutting down Mercury before HIERARCH.PM has saved properly. It takes a minute or two on my system. The lock file MAILBOXM.LCK in the same folder disappears at that time.

The fix is as Thomas says (as is everything else!).

 Chris

I have suddenly lost all my trays  in the 'work with mail folders' list and all the folders now appear in one list in the order of begining with 0-9 followed by a-z. How can I restore the trays, please without having to do it all manually. Thanks.

> I have suddenly lost all my trays  in the 'work with mail folders' list

and all the folders now appear in one list
> in the order of beginning with

0-9 followed by a-z. How can I restore the trays, please without having

to
> do it all manually. Thanks.

Exit WinPMail and restore the file hierarch.pm from your last backup.  If you have no backup then this rebuilding of the trays has to be done manually.