if you are able to enter grub advanced options then you can get into manjaro with any available kernel. if not present then boot with a live usb:
sudo manjaro-chroot -a
pacman -Syyu
mhwd-kernel -i linux54 #install the 5.4 lts
mhwd-kernel -li #verify if correctly installed
exit #end-chroot
side note if you are not able to see grub on boot then:
sudo manjaro-chroot -a
vim /etc/default/grub
#change GRUB_TIMEOUT_STYLE from hidden to menu
update-grub #this is a command
exit
reboot
