I set all of my fields in one list screen to have a return screen value of 0, and now I can't access my database.

To fix this issue, you need to set at least one return screen value to something other than 0. The directions for this through Navicat are as follows:

1. Find out the tableId of the table in the DatabaseTables table that is causing problems.
2. In the DatabaseFields table use the Filter Wizard to filter for that tableId. It will look something like this tableId is equal to 43
3. Set the return screen order of one of the fields to something other than 0.
4. The database should now work.

Alternativly you can run the following sql command:

UPDATE DatabaseFields,DatabaseTables
SET DatabaseFields.returnScreenOrder = [new return screen order]
where DatabaseFields.fieldName='[field name]' and DatabaseFields.tableId = DatabaseTables.tableId and ->DatabaseTables.tableName='[table name]'