Your monitor is black with a white cursor flashing in the upper left corner -- not the colorful log-in screen you expected. Something is wrong, but your computer isn't indicating what. The fact that your operating system didn't load is, in itself, a clue. You'll have to do some more digging to discover what's wrong.
Your computer may simply have a "hiccup." Shut down your computer completely, disconnect power and wait 30 seconds. If your computer reboots properly and the blinking cursor issue is a rare occurrence, you may have no problem.
USB flash drives, hard drives or other peripheral devices may interfere with your computer's booting. Disconnect any devices except keyboard and mouse, particularly wired devices connected directly to USB ports. If your computer boots properly, try reconnecting each USB device individually to see which one causes your issue. This problem is more common in laptops, but it can occur on desktops as well.
Hard Drive Failure
When nothing else boots the computer, it may mean your motherboard no longer recognizes the hard drive with boot information. You can confirm this by entering your computer's BIOS upon powering up to see if the hard drive is connected. If it's not connected, this may mean the drive's cable is loose or that the drive itself is no longer working.
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