i guess that happened after making a ubuntu kernel update , so you updated the kernel but not updated grub config , grub tries to boot the old kernel file but it's removed
so let's boot manually from grub command line
first try ls (hd0,msdos1)/ and ... and find your /boot partition (or if all in one partition find your ubuntu partition)
when you find "/boot"(if you have separate partition for it) or "/"
enter : set root=(hd0,msdosX)
this will set the root browsing directory for grub (replace X)
look for a file haveing "vmlinuz" in beginning of it's name
and a file having "initrd" in beginnig of its name (they should be on the same folder in /boot or its subfolders, if there are multi files with that names select a pair that have the same version in their name (last is best ) )
now we suppose the files you've found are :vmlinuz-4.19.0-kali4-amd64 & initrd.img-4.19.0-kali4-amd64
in grub command line enter :
linux /boot/vmlinuz-4.19.0-kali4-amd64
initrd /boot/initrd.img-4.19.0-kali4-amd64
boot
congratulations you booted your ubuntu(or any other unix) successfully from grub command line manually and should see your OS starting up if you've done these step correctly