Go to IT0007 and click on CTRL+F7 then click on SHIFT+F7 rpeat till you get the last month work schedule where finally you will get a message saying that work schedule deleted or does not exist for XX-XX-XXXX i.e it is not generated or not exist.
Go to table V_T508A and check the work schedule rule till what date it is valid.
Regards
Venkatesh