boot - How to recover broken Ubuntu 18.04 LTS install having been dd'd over by iso9660?

I have a Dell laptop with a 120GB SSD hard disk. It has Ubuntu 18.04 LTS installed on a single partition on the main drive.

Whilst trying to make a USB boot disk from the command line, I accidentally did a umount /dev/sda1 or /dev/sda (can recall). Shortly after the machine spewed some log errors and hung.

When I boot the machine, I've taken to the grub rescue prompt, which displays "error: unknown filesystem. Entering rescue mode...".

Using a friend's computer, I created an Ubuntu 18.04 LTS usb live pen. From the "try without installing" option, I was able to get access to Wifi and install boot-repair tool. (see output below, with paste bin link at the bottom).

It appears that Grub sees the file system incorrectly as iso9660. When I installed Ubuntu using the GUI installed, I chose the default options (so I'm not sure whether it should be ext2, ext3 or ext4).

Either way, it's most certainly not iso9660. How do I repair the Grub configuration so that I can boot from the hard disk please?

Update: I have since tried booting to the ubuntu USB drive and manually mounting the /dev/sda1 120GB SSD. It is in fact a iso9660 fstype containing the Ubuntu installer, so it would appear I dd copied over my OS with the 1.89GB installer image.

I've obviously lost the first ~2GB of data, but is there a way to recover the rest of the file system?

---

sda1:
File system: iso9660
Boot sector type: Unknown
Boot sector info:
Operating System:
Boot files: /boot/grub/grub.cfg

Drive: sda _____________________________________________________________________
Disk /dev/sda: 119.2 GiB, 128035676160 bytes, 250069680 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Partition  Boot  Start Sector    End Sector  # of Sectors  Id System
/dev/sda1    *          2,048   250,068,991   250,066,944  83 Linux
Device           UUID                                   TYPE       LABEL
/dev/loop0                                              squashfs
/dev/loop1                                              squashfs
/dev/loop2                                              squashfs
/dev/loop3                                              squashfs
/dev/loop4                                              squashfs
/dev/loop5                                              squashfs
/dev/loop6                                              squashfs
/dev/loop7                                              squashfs
/dev/sda1        2018-07-25-03-21-56-00                 iso9660    Ubuntu 18.04.1 LTS amd64
/dev/sdb1        B1DB-1D1B                              vfat       UNTITLED

http://paste.ubuntu.com/p/4PRRWF7r6B/