I am trying to set up a multi-boot on my machine with Ubuntu (my original OS, on /dev/sda2), Kali Linux and Debian. However, I got stuck halfway through my installation of Debian, and since Ubuntu took a lot of time to boot, I followed the steps of this post to make the boot process faster. But when I rebooted my machine, Ubuntu would only boot in emergency mode... The only thing I was able to notice was that in my /etc/fstab the line associated with my Ubuntu partition was gone.

I would gladly post the contents of my fstab here but I don't know how to copy it from the emergency mode to here (I am using my Kali Linux on /dev/sda5 to write this post). Maybe there is a way to restore my fstab, to begin with?

Edit 1

Here is the content of my /etc/fstab:

# /etc/fstab: static file system information.

#

# Use 'blkid' to print the universally unique identifier for a

# device; this may be used with UUID= as a more robust way to name devices

# that works even if disks are added and removed. See fstab(5).

#

# <file system> <mount point>   <type>  <options>       <dump>  <pass>

# /boot/efi was on /dev/sda1 during installation

UUID=95B2-5AED  /boot/efi       vfat    umask=0077      0       1

# /home was on /dev/sda3 during installation

UUID=69d6623e-0bcc-4cef-8b25-e46c98210d44 /home           ext4    defaults            0       2

# swap was on /dev/sda4 during installation

UUID=a8ee0943-0cd9-4dba-b018-ca00fc450e5d none            swap    sw              0       0

And here is thd result of blkid | grep UUID:

/dev/sda1: UUID="95B2-5AED" TYPE="vfat" PARTLABEL="EFI System Partition" PARTUUID="f3ead83c-a7ca-453b-8317-a854080d37fc"

/dev/sda2: UUID="7d4d2f18-146c-4d56-b5f3-0dc605eeb9e0" TYPE="ext4" PARTLABEL="Ubuntu" PARTUUID="94d6c9bd-30da-4abf-a784-41e20992fdd4"

/dev/sda3: UUID="69d6623e-0bcc-4cef-8b25-e46c98210d44" TYPE="ext4" PARTLABEL="Home" PARTUUID="dd1299b6-adb1-45c0-99a6-94e922f4964b"

/dev/sda4: UUID="a8ee0943-0cd9-4dba-b018-ca00fc450e5d" TYPE="swap" PARTUUID="228fa2d0-8b0c-4562-bb5a-ebb73bb00f04"

/dev/sda5: UUID="489b70a2-db82-4b0c-bebd-cf19a403ade1" TYPE="ext4" PARTUUID="48ba997c-e595-45c1-93c0-b97e4f7ffbf5"

/dev/sda6: UUID="9068da24-6073-45dc-a18e-29634daa3910" TYPE="ext4" PARTUUID="9033f352-349f-4cee-94bf-c686f462adea"

Edit 2

I ran the e2fsck command on my Ubuntu, home and Debian partitions, and now instead of booting into emergency mode, Ubuntu starts to launch normally, but freezes after some time loading.

Since your Kali installation is working, you can use it to access your Ubuntu installation in a chroot. To do this, run the following commands as root:

mkdir /ubunturoot

mount /dev/sda2 /ubunturoot

mount -o bind /dev /ubunturoot/dev

mount -o bind /dev/pts /ubunturoot/dev/pts

mount -o bind /proc /ubunturoot/proc

mount -o bind /sys /ubunturoot/sys

chroot /ubunturoot

Now your command prompt window (note: this particular shell only!) should be accessing your Ubuntu root filesystem just as if you had logged onto Ubuntu and become root in Ubuntu. Take a look and ensure everything is as it should be.

If your Ubuntu /etc/fstab is in error, now you can edit it.

Once that is fixed, first make sure the /boot/efi filesystem is mounted in your Ubuntu chroot:

mount /boot/efi

Then run ls /lib/modules to see one or more directories named with kernel version numbers. Use update-initramfs -u -k <kernel version number> to update the initramfs file of the respective Ubuntu kernel. (Since you are now really running Kali's kernel, you must explicitly specify the version number of Ubuntu's kernel: trying to update the default kernel would result in an error message since Ubuntu's and Kali's kernel versions are unlikely to match.)

Then check /etc/default/grub for boot options mentioning filesystem UUIDs or other things that may have changed on your OS installations. Fix as necessary, then run update-grub to update the configuration file of Ubuntu's GRUB bootloader.

Once you've fixed all the problems you've found, undo the temporary chroot environment manually:

umount /boot/efi

exit # out of the chroot environment, back to Kali native view of the filesystem

umount /ubunturoot/sys

umount /ubunturoot/proc

umount /ubunturoot/dev/pts

umount /ubunturoot/dev

umount /ubunturoot

rmdir /ubunturoot