I find this happens to me from time to time.
Dreamweaver seems very fussy if (valid) changes are made to a SQL statement, but the syntax doesn't exactly comply with its own preferences.
Sometimes you get a big red exclamation point, and occasionally the recordset seems to disappear altogether.
If the page still works ok, you could hand code changes to the SQL or alternatively try backing up your page, then deleting the recordset and adding it again via the interface