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PC won't start: step-by-step diagnosis

Nothing lights up, black screen, stuck on the logo or Windows no longer loads: identify the symptom and the likely cause, in the right order.

Hardware Tutorial Beginner Published October 19, 2026 Translated October 27, 2026 10 min read Version française
Installing a component in an open PC
Photo: JESHOOTS.COM on Unsplash

Before you start

  • Another device (a phone) to read this guide while troubleshooting

In plain words

A PC starting up is a rocket lifting off in stages: power, then hardware, then the disk, then Windows. Spotting the stage where the rocket stops is enough to rule out most suspects. No point suspecting Windows if even the fans are not spinning.

A PC that will not start is not necessarily lost. The method is to watch how far it gets before stopping: each stage it passes rules out causes.

What you see Go to
Nothing at all: no light, no fan Step 1
The fans spin, but the screen stays black Step 2
The manufacturer's logo appears, then nothing or a message Step 3
Windows starts loading then fails, restarts in a loop or shows a blue screen Step 4

Safety

Before opening a desktop PC, unplug the power cable and touch a metal part of the case to discharge static electricity. Never open a power supply unit. On a laptop still under warranty, do not open the machine: contact the manufacturer.

Step 1: no sign of life

The cause is almost always electrical.

  1. The socket: test it with another device. Plug the PC straight into the wall, without a power strip.
  2. The power supply switch, at the back of a desktop PC: it must be on I.
  3. Laptop: plug in the charger and check its light. Let it charge for 30 minutes if the battery was flat.
  4. Full discharge: unplug the cable (and the battery if it is removable), hold the power button for 30 seconds, plug back in and try again. This is often enough after a power cut.

Still nothing on a desktop PC? The power supply is the first suspect. It is an easy part to replace.

Step 2: fans spinning, black screen

  1. The monitor: is it on, on the right input (HDMI, DisplayPort)? Try another cable.
  2. Desktop PC with a graphics card: the monitor must be plugged into the graphics card, not into the motherboard outputs.
  3. Beeps or lights: many motherboards report the fault with a series of beeps or lights labelled CPU, DRAM, VGA, BOOT. Note what lights up and look up the meaning in the manual.
  4. The memory (RAM): the most common cause. Remove the modules, then push them back in firmly until they click. Try with a single module, then the other.
  5. Reset the BIOS: remove the coin battery from the motherboard for a minute (PC unplugged), or use the "CLR_CMOS" jumper described in the manual.

Step 3: logo shown, then stuck

The main hardware works. The PC cannot find, or cannot read, the boot disk.

  1. Unplug everything USB (sticks, external drives, printer): the PC may be trying to boot from it.
  2. Enter the BIOS/UEFI (Del, F2, F10 or F12 at startup depending on the brand) and check that the disk appears in the list. - Disk missing: unplugged cable (desktop PC) or failed disk. - Disk present: check the boot order; the system disk must come first (often called "Windows Boot Manager").
  3. A message such as "No bootable device" or "Boot device not found" confirms a disk or boot order problem.

Step 4: Windows does not load

After two failed starts in a row, Windows opens the recovery environment (blue "Choose an option" screen). Go to Troubleshoot → Advanced options and try, in this order:

  1. Startup Repair: fixes startup automatically.
  2. Uninstall Updates: if the problem appeared right after an update.
  3. System Restore: goes back to an earlier restore point, without touching your documents.
  4. Startup Settings → Restart, then press 4 for Safe Mode: Windows starts with the minimum, so you can uninstall a faulty driver or program.

As a last resort, resetting Windows is covered in Repairing or resetting Windows 11.

Blue screen

Take a photo of the blue screen: the stop code shown (for example CRITICAL_PROCESS_DIED or INACCESSIBLE_BOOT_DEVICE) points the search towards the cause.

Save your data first

If the disk is detected but Windows no longer starts, your files are probably intact. Before any reinstall, recover them by booting from a live Linux USB stick (Ubuntu, for example), which lets you copy files to an external drive without installing anything.

BitLocker-encrypted disk

Many recent PCs encrypt the disk out of the box. To read your files from another machine or a rescue system, and sometimes just to enter the recovery environment, you need the BitLocker recovery key (48 digits). It is usually saved in the Microsoft account: look it up from a phone at aka.ms/myrecoverykey.

Sources

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