Is a single blue screen serious?

Rarely. A blue screen means Windows ran into something it could not handle and restarted to be safe. If it happens once and the computer then behaves normally, keeping an eye on it is enough. If it comes back, there is a cause worth tracking down.

What is the stop code and why write it down?

The stop code is the line in capitals on the screen, such as MEMORY_MANAGEMENT or CRITICAL_PROCESS_DIED. It points to which part of the computer was involved, and it is what makes the cause findable instead of guesswork.

Does a blue screen delete my files?

No, the blue screen itself deletes nothing. But repeated blue screens can be a sign of a drive on its way out, so copying your photos and documents to a USB stick is something to do today rather than next week.

Blue screen in Windows? How to find what is causing it

A blue screen means Windows ran into something it could not handle and restarted to be safe. The computer almost always comes back on its own. The first thing to do is photograph the text on the screen, because it contains the code that decides what the cause is.

If it happens once, keeping an eye on the machine is enough. If it comes back, work through the steps below.

Try this first

Photograph the screen with your phone. Otherwise copy the text down. What matters is the line in capitals with underscores, such as MEMORY_MANAGEMENT or VIDEO_TDR_FAILURE, plus the name of a file if one is shown.
Let the computer restart. It counts up to a hundred per cent and then restarts on its own. Do not interrupt it, and do not switch off with the power button while it is counting.
Think about what changed in the last few days. A new printer, a new program, a new update, new memory, a new graphics card. Whatever was added last is the first thing to take away again.
Unplug the extras. Disconnect everything except the screen, keyboard and mouse: USB sticks, external drives, printers, webcams, card readers. Use the computer for a day like that. If the blue screens stop, one of those was involved, and you can plug them back in one at a time.
Run Windows Update. Start, Settings, Windows Update, Check for updates. Install everything offered and restart. A large share of blue screens come from outdated drivers, the small programs that let the graphics card and other parts work, and this is where they are updated.
Update the graphics driver. Get it from Intel, AMD or Nvidia depending on what the computer has, or from the manufacturer's support page for your exact model. Never download a driver from a site that appeared in an advert.

If that did not help

Copy your files off first. Before going further, put your photos and documents on a USB stick or an external drive. Repeated blue screens can mean a drive on its way out, and then the copy is urgent.
Test the memory. Press the Windows key, type Memory Diagnostic and open Windows Memory Diagnostic. Choose to restart and check now. The computer restarts and shows a blue test screen for a few minutes. When it comes back, the result appears as a notification at the bottom right.
Check the drive. Press the Windows key, type Command Prompt, right-click it and choose Run as administrator. Type chkdsk /f and press Enter. Answer Y when asked whether to run it at the next start, then restart. The check takes a while and deletes no files.
Repair Windows own system files. In the same black window, type sfc /scannow and press Enter. Let it finish without closing the window. It takes around ten minutes and does not touch your documents.
Clear dust from the fans. Overheating gives blue screens that arrive once the computer has been working a while. A desktop can be blown clean with compressed air while it is switched off and unplugged.
If the blue screen arrives before Windows even starts, read the guide on Windows not starting and use Startup Repair from there.

When to ask a person

If blue screens come several times a day, if the memory test finds something, or if the computer makes a clicking or scraping sound, stop trying things yourself. That points to a physical part inside the machine, and it needs replacing rather than adjusting.

Have the stop code ready when you ask. With that code and a description of what you were doing when it happened, the cause can usually be identified straight away.