The system will not boot

A machine that will not boot is not always a dead disk. Often it is GRUB, a full disk, or a kernel that cannot find the root filesystem. Work in this order.

Can you reach GRUB?

Hold Shift (BIOS) or press Esc (many UEFI boxes) at power-on. If the GRUB menu appears, open Advanced options and boot the previous kernel. If that works, the newest kernel or its NVIDIA/Wi-Fi module is the problem. Update later from a running system; do not keep hammering the broken kernel.

GRUB rescue prompt

If you see grub rescue>, GRUB cannot find its files. A live USB (Ubuntu desktop ISO) is the reliable fix: boot it, install boot-repair or remount your root and reinstall GRUB as documented for your OS. Guessing insmod commands from memory is how people delay a 15-minute repair.

Emergency / busybox / “failed to mount”

If you get a root shell or busybox, the disk is probably there but dirty or out of space.

df -h
lsblk
sudo fsck -n /dev/sdXN

Replace sdXN with the root partition from lsblk. Read the fsck man page for your filesystem. fsck on a mounted disk is dangerous; from a live USB, run it on the unmounted root partition. Journals that fill / also prevent boot; from a live environment, delete old journals only after you confirm you are on the right disk.

When it is the disk

Clicking, SMART errors, or the drive missing from lsblk on a live USB means hardware. Recover files first if the disk still appears. Do not keep running fsck in a loop on a dying drive.

If this is a VPS, use the provider’s rescue image instead of a USB. Same idea: mount the disk, read logs, fix GRUB or fstab.