Warning
Use a sudo-capable account, keep shell history for audit, and back up critical paths before changing boot, SSH, or package state.
What you will achieve
Work through boot failures methodically — from kernel panic to fstab typos.
1) Use GRUB advanced options
Boot older kernel or recovery mode if current kernel fails.
2) Repair filesystem and initramfs
sudo fsck -f /dev/sdXY
sudo update-initramfs -u3) Check failing services after boot
systemctl --failed
journalctl -b -p err --no-pager