Windows may fail to start for a variety of reasons, and generally speaking in order of decreasing likelihood here they are:
Corrupt file or volume
Let me elaborate. A common reason systems fail to start is because some element of the system's hardware has failed. This could range from the simple (someone kicked the power cord out of its socket) to the obvious (smoke emitting from the machine) to the mysterious (something transient that happens only when the moon is full or during sunspot minimum). Next most common is when you update the driver for some piece of hardware (or the BIOS for that matter) and the system won't boot afterwards. After that comes those mysterious messages we'll talk about shortly that usually indicate some key operating system file has somehow become corrupt or gone missing. Misconfiguration is another possible source of boot problems, but this is somewhat rare as in most cases you'll still be able to boot but one or more services may fail to start or your applications may not function as expected. Finally, virus infection can cause a system to fail to boot, but I've listed this in last place because I'm assuming you've got an antivirus solution in place and you're keeping the antivirus signature files updated, right?
Now that we know why Windows may fail to start properly, let's ask the logical next question: How can we know which of these underlying causes is the one that might be preventing Windows from successfully booting?
How to diagnose startup problems
Here is where we need to apply our brains and use a bit of common sense to determine what the cause of startup failure might be. Think of the previous list above as a list of disease-causing viruses, and now you have to play doctor and figure out which virus the patient (your sick computer) actually might have. For if you skip this step and try blasting the patient with every possible remedy in your doctor's bag, two things may happen:
One of the remedies you try may actually make the patient worse and indeed could prove fatal.
You'll waste a lot of time and the recovery of your patient will be delayed, and your boss may get upset with you as a result since her business is losing money due to downtime.
So careful diagnosis is a step you should always take time for and never avoid, and just like in the medical profession such diagnosis usually begins with your senses. For example, do you smell something burning? Better unplug your system immediately and wait for things to cool off, then open the case and inspect the damage. Do you hear your CPU fan making a slow grinding sound? Power down your system and replace the fan before your processor burns out and needs replacing. Is your video display flickering? Maybe try reseating the video card after checking if the video cable is seated properly.
OK let's assume its not such a simple and obvious problem. Instead, say you get a black screen with one of the following dreaded messages when you try and boot your system:
"NTLDR is missing"
"A disk read error occurred"
"Invalid partition table"
"Error loading operating system"
"Could not read from selected boot disk"
"Windows could not start because the following file is missing or corrupt"
Or you might get a blue screen (called a STOP screen) with some obscure message on it. Or if you're lucky you might make it all the way through the Windows splash screen to the logon box and then suddenly get a dialog box saying "One or more services failed to start". Or your mouse pointer might freeze and your system hang either before or immediately after logon. How can you match these symptoms to the underlying condition that might be causing them? First let's look at some possible "black screen" messages that can occur after the BIOS POST routine finishes but before the Windows splash screen appears:
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