Greetings,
There are a number of potential reasons for this error:
1) Make sure ListMail files correspond with your database version. For instance, if you've upgraded to v1.85 you cannot use the v1.84 files.
2) Check that your MySQL server is not down temporarily. Do you get this error from any other ListMail pages? Has the error gone away since you posted?
3) You may need to do a manual "REPAIR TABLE lm_config" SQL statement to repair your lm_config table.
It could also be something more serious, such as your host losing your entire database due to a server crash and not keeping proper backups. This is rare, however.
Regards