Dear Sandeep,
As Mr.Prasoon suggested kindly refer the note 895449, The issue because of following condition listed below.
1. If you are using Central purchasing Organization. (or)
2. If the condition record validity period just got expired. (or)
3. If the master condition still valid while run ME59N if info record available for existing last PO.
Kindly try the same...
Regards,
Karna J