Grub - lost access to Windows 10 partition

Question

I used to be able to choose between booting Ubuntu(default) or using the arrow keys to go down to and pick Windows 10. At some stage, the Windows 10 option simply disappeared - but Ubuntu continued to work fine.

I'd like to recover my Windows 10 partition and use it.

What I've tried so far is:

boot-repair - strangely, in the report, I have mentions of a Windows partition:

sda2: __________________________________________________________________________

    File system:       ntfs
    Boot sector type:  Windows 7/2008: NTFS
    Boot sector info:  According to the info in the boot sector, sda2 has 
                       3130845183 sectors, but according to the info from 
                       fdisk, it has 11720779775 sectors.
    Operating System:  
    Boot files:

So far so good. Looking into the partitions, however, sda2 doesn't seem quite right:

sudo lsblk -o NAME,FSTYPE,SIZE,MOUNTPOINT,LABEL

sda              5.5T                                    
├─sda1 ext4      128M                                    external-drive
└─sda2 ntfs      5.5T /mnt/usb-Seagate_Expansion_Desk_NA Seagate Expansion Drive
sdb            238.5G                                    
├─sdb1 vfat      512M /boot/efi                          
└─sdb2 ext4      238G /                                  
sdc            931.5G                                    
└─sdc1 ext4    931.5G                                    
sdd            223.6G                                    
├─sdd1 ext4    215.6G /mnt/boot-sav/sdd1                 
├─sdd2             1K                                    
└─sdd5 swap        8G

sda2 appears to be my external hard drive I use to share data between Ubuntu and Windows(for running Plex, amongst other things).

Any suggestions as to how I recover it? Boot-repair doesn't seem to have much of an effect, unfortunately.