If your Windows partition is still there then you can run
and if GRUB can find the Windows boot loader then it will add it to the GRUB menu.
It might be possible that Ubuntu and Windows are not installed with either or both being the legacy boot mode or both being UEFI. If one is UEFI and one is legacy BIOS then GRUB will not be able to populate the menu with windows 10.
Another option is to use the BIOS boot options menu, usually press F12 or some similar key on boot up to access it, and select the Windows partition to see if you can boot from that partition directly.
If your Windows partition is gone then you only option may be to reinstall Windows and then install Linux again making sure everything installs to where they are supposed to be. If you have access to another Windows computer you can prepare a Windows installation USB/CD using this program on microsoft.com. Make sure you install both OSes as BIOS or as UEFI.