Interrupted upgrade can't log on

What exactly is happening when you boot and try to log on?

When I try and log on I seem to be in the windows partition.
I can log onto Windows but no trace of the Linux partition.
I can go down to the command prompt but it is in windows x:\windows\system32>
Do I need to do a clean install?

Do you see a grub menu when booting?
If yes, what choices does it show?

No grub menu

It would be good to see the layout of your disks and the related operating systems.
This information can e.g. be obtained by running the "info" mode of boot repair.
In your case you would probably have to start it by booting a boot-repair disk.
See https://help.ubuntu.com/community/Boot-Repair

Maybe better link with explanation:
https://help.ubuntu.com/community/Boot-Info

Thank you for your help.
I don't have the laptop at present as someone is trying to recover the data.
When I get it back I will try this procedure and let you know what happens.

Thank you for your help but I still have problems.
When I got the laptop back I tried booting it with a liveUSB but could not turn on networking so could not connect to the internet.
As I had my data backed up I reinstalled Ubuntu 18.04.2LTS from the live-USB but when I restarted the laptop it booted up in Windows10!
Meanwhile I had downloaded the iso of Boot repair disk 64 bit iso from Yannubuntu but cannot yet it onto a USB.
Tried using UNetbootin but could not get the target correct.
Can you suggest another program?

Did a clean install of Windows 10 and then installed Ubuntu 18.04.2 to run alongside it.
Now when I start the notebook it boots up in Windows without offering me the choice of of Ubuntu or Windows in the Grub menu.
Any suggestions on how I can restore Grub?

I do not understand why the Ubuntu installer has not created the grub menu to select between Ubuntu and Windows.
Apparently you have made a wrong choice for one of the questions of the installer.

My suggestions:
Either try boot-repair
or try https://www.omgubuntu.co.uk/2011/09/live-usb-sticking-grub-2-video

Thank you for giving me the link.
All went well once I had realized that there were two dashes before the --bind commands but at the grub-install stage I hit a roadblock.
Here is the output:
ubuntu@ubuntu:~$ sudo chroot /mnt
root@ubuntu:/# grub-install /dev/sdb1
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.
root@ubuntu:/# grub-install /dev/sdb
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.
root@ubuntu:/# grub-install /dev/sda1
Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.
root@ubuntu:/# grub-install /dev/sdb1
Installing for x86_64-efi platform.

grub-install: error: cannot find EFI directory.
On the notebook the Linux partition is dev/sda5 and the boot partition is dev/sdb1
What point have I missed.
How do I run boot-repair your other suggestion?

1. What is the layout of your hard disks? It might be that you selected several possible parameters, but not the right one
grub-install /dev/sda

Don't confuse the /boot directory of Ubuntu with the "boot disk" (which is the hard disk where the BIOS searches for operating systems to boot).

2. For boot-repair and boot-info see the links that I gave in an earlier answer:

https://help.ubuntu.com/community/Boot-Repair
https://help.ubuntu.com/community/Boot-Info