The computer makes noise but the screen stays black, what does that mean?

It means the computer has power but the picture is not reaching the screen. Check that the screen is switched on, that the cable is firmly in at both ends and that the screen is set to the right input. On a desktop, try moving the video cable to the other socket at the back.

Does Startup Repair delete my files?

No. Startup Repair only fixes what Windows needs in order to load, and leaves your documents and photos alone. Reset this PC is an entirely different option that can erase everything, so it should never be run without a copy of your files.

How do I get to my files if Windows will not start?

Through Safe Mode, reached via Advanced options and Startup Settings. From there you can usually copy photos and documents to a USB stick. If that is not possible either, a technician can connect the drive to another computer and read the contents out.

PC will not start? How to find where it stops

Start by working out what the computer actually does when you press the button. Is it completely silent, no lights and no sound? Or does it start, make noise, and then stop before Windows loads? The two situations have entirely different answers, and it takes ten seconds to see which one you have.

Read the section that matches your computer.

Try this first

If the computer is completely silent, no lights and no sound:

Check the power at both ends. The cable should be firmly in the computer and in the wall socket. If it runs through an extension block, check that the block's own switch is lit. Test the socket with a lamp you know works.
Charge a laptop for an hour and then try again. A fully flat battery sometimes needs a while before the computer will start at all.
Drain the residual power. Unplug the charger, hold the power button down for thirty seconds, release, plug the charger back in and press start.
Try a different charger if you can borrow one that fits. A failed charger looks exactly like a failed computer.

If the computer starts but Windows does not load:

Unplug everything in the USB ports. USB sticks, external hard drives, card readers and phone cables. The computer may be trying to start from the wrong device and then simply waits. Restart.
Check the screen. That it is switched on, that the cable is in at both ends, and that it is set to the right input. A black screen looks just like a dead computer.
Let Windows do its own repair. If Windows gets halfway and stops three times in a row, it opens a blue menu called Automatic Repair by itself. Let it finish.

If that did not help

Run Startup Repair. In the blue menu, choose Troubleshoot, then Advanced options, then Startup Repair. Pick your account and type your password when asked. The computer works for a few minutes and restarts. This deletes no files.
Try System Restore. Back in Advanced options, choose System Restore and then a point in time from before the computer stopped starting. Your documents and photos are untouched, but programs installed after that point are removed and need installing again.
Reach your files through Safe Mode. Advanced options, Startup Settings, Restart. When the list appears, press 5 for Safe Mode with Networking. Windows loads with only the essentials. Plug in a USB stick and copy across anything you want to be sure of keeping.
If you cannot reach the blue menu at all, press the power button as soon as the Windows logo appears, so the computer switches off mid-start. Do that three times. The fourth time, the menu opens on its own.

The one step not to take alone

The same menu contains Reset this PC. That option can erase everything on the computer, and it is the step that most often ends with photos gone for good. Never run it before your files are copied to a USB stick, an external drive or OneDrive, and before someone has confirmed the copy actually opens.

If you hear clicking or scraping from inside the computer, switch it off and stop trying. That sound comes from a mechanical hard drive giving up, and every further attempt to start reduces the chance of recovering what is on it. The right order then is to have the files read out first and repair the computer afterwards.

Common reasons a computer stops starting are a power cut during an update, a drive beginning to fail, or simply a USB stick left plugged in.