Windows restart without warning
Software issue or error
Since the release of Windows XP, Microsoft designed Windows to automatically reboot when errors occur, such as a BSoD. Although this feature is nice for random errors, it does make it difficult to troubleshoot or repair. Below are the steps on how this feature can be disabled and enabled in Windows XP, Vista, 7, 8, and 10.
From the desktop, right-click My Computer. In Windows 8 and 10, open File Explorer and right-click This PC in the left navigation pane.
Select the Properties option in the pop-up menu.
In the System Properties window, click the Advanced tab or the Advanced system settings link.
On the Advanced tab, click the Settings button under Startup and Recovery.
In the Startup and Recovery window, uncheck the Automatically restart check box.
Click OK.
Now, if the computer generates an error, it should not automatically restart and should display an error message, most likely a BSoD.
Hardware issue or error
Any failing hardware component in your computer could cause your computer to generate an error or reboot without warning. If you have recently attempted to add a new hardware device, remove that device to make sure it is not causing your issues.
Make sure you are not encountering any hardware conflicts by viewing Device Manager for errors.
Hardware drivers can also cause this issue. If you have recently installed new hardware drivers, you may want to try an earlier version of those drivers.
If you updated drivers and removed any newly added hardware and the random reboots continue to occur, the computer memory is bad.
Finally, hardware devices that are not installed properly in the computer can also cause random reboots. Make sure all the cables and expansion cards are connected properly inside the computer. The best way to determine this is to disconnect and reconnect all cables and expansion cards.
Computer virus
Some computer viruses, such as the blaster virus, are designed to reboot your computer without warning. Usually, these viruses make the computer reboot every 5, 10, 15, or 30 minutes after the computer has booted.
If you believe your computer has a virus, make sure an antivirus program is installed and the virus scanner definitions are up-to-date.
Heat-related issue
Most computers today are designed to turn off or automatically reboot if the computer processor, video card, or other devices in the computer get too hot. If you have heard any abnormal noises coming from your computer recently, it could be a sign of a failing fan, which can cause hardware components to overheat.
First, verify the fan on the power supply is working by examining the back of the computer. Check if the fan is moving, moving fast, and moving smoothly (e.g., not wobbling or scraping against the insides of the power supply). For all other fans in the computer, you need to open the computer and verify all fans are working (e.g., processor fan, video card fan, and case fans).
Issue with the operating system
If, after following the recommendations above, your computer continues to reboot, you're likely experiencing an issue with the Microsoft Windows operating system. To help determine if this is the case, try the steps below.
Reboot the computer and enter BIOS setup when the computer begins to boot up.
After you have loaded the computer into the BIOS setup, let the computer idle for a few minutes.
If the computer doesn't reboot after idling in BIOS and you've tried the other solutions above, try activating a restore point in Windows before the problem started to occur. If activating a previous restore point does not work, or there are no restore points available to activate, you likely need to reinstall Microsoft Windows.