i did this :
Type bootrec /rebuildbcd and then press ENTER
• If Bootrec cannot locate any missing Windows installations, you must remove the BCD store, and then you must re-create it. To do this, type the following commands in the order shown below: (Remember to press ENTER after each command)
bcdedit /export C:\BCD_Backup
ren c:\boot\bcd bcd.old
bootrec /rebuildbcd