The ordinal position for a column changed without my knowledge
Someone changed the table, simple as that. Maybe not on purpose, e.g. they could have been changing other things in the mickey mouse table designer and accidentally clicked and dragged a column up or down. Figuring out who and when will be guesswork unless you have auditing in place, or the ALTER or DROP/CREATE events are still in the default trace:

DECLARE @path NVARCHAR(260);

SELECT 
   @path = REVERSE(SUBSTRING(REVERSE([path]), 
   CHARINDEX(CHAR(92), REVERSE([path])), 260)) + N'log.trc'
FROM    sys.traces
WHERE   is_default = 1;

SELECT 
  LoginName, HostName, ApplicationName, StartTime, Action = CASE EventClass 
    WHEN 46 THEN 'Create' WHEN 47 THEN 'Drop' ELSE 'Alter' END
FROM sys.fn_trace_gettable(@path, DEFAULT)
WHERE EventClass IN (46,47,164)
AND EventSubClass = 1
AND DatabaseName = N'your database name'
AND ObjectName = N'your table name'
ORDER BY EndTime DESC;