Hello, Vivekanand Gaur
Welcome to the Microsoft community.
Thank you for your feedback. I understand that you are experiencing a dual boot issue. Here are some troubleshooting options:
Open a command prompt as administrator in Windows 8
Runbcdedit /enum
check that the entries for Windows 10 exist and are correct
bcdboot X:\Windows /s C: /f ALL
where X is the drive letter where Windows 10 is located
Check the partition labeling:
Type active to ensure that the partition where Windows 10 is located is correctly marked as active
Check Hard Disk Mode:
Verify in the BIOS that the hard disk mode (AHCI/IDE) matches the mode used when installing Windows 10
Reinstall the Windows 10 bootloader:
Boot your computer using the Windows 10 installation media
Select “Repair your computer” and then use the command prompt to
Run
bootrec /rebuildbcd
bootrec /fixmbr
bootrec /fixboot
Check the disk order:
Verify that the boot sequence is correct in the BIOS
If none of the above methods work, you may want to consider reinstalling Windows 10. in this case, make sure you back up all your important data first.
If the problem persists,it is more suitable for publishing on Microsoft Learn, more users post these issues there, you can click on "Ask a question", there are experts who can provide more professional solutions in that place.
Here is a link to the forum where you can raise specific scenarios and share your idea to help solve the problem.
I look forward to your feedback.
Best Regards,
Aaron| Microsoft Community Support Specialist