If your laptop suddenly shuts itself off for no apparent reason, there are several factors that can contribute to the problem. When you troubleshoot the laptop, remember that it is also possible for a combination of causes to be at work.
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All newer laptops have built-in heat sensors that shut the computer off when it exceeds a pre-set temperature threshold. Install and run a temperature monitoring utility to determine whether the laptop is overheating.
If you replaced or installed new hardware in your laptop, use the device management utility to review if there are any resource conflict or failure reports. You can also remove unnecessary hardware and run the laptop to see if the problem persists. If it doesn't, reinstall each hardware component and re-test until the laptop fails.
Operating System Issues
The operating system start-up controls can affect how the laptop performs. Follow the operating system developer's instructions for entering the start-up program, and confirm that the settings are what your laptop requires.
If your laptop is not getting the correct power it requires, it will shut down completely. Test the current of any outlets you use with your computer, employ a surge protector to ensure steady current, and confirm that the power supply itself is not defective.
Failing batteries can produce unexpected laptop shutdowns. Use a battery monitoring utility to determine whether or not the battery is healthy and replace it if it isn't.
Unfortunately, rootkits and viruses can cause your laptop to shut down without warning. You will probably have to take the hard drive out of the laptop to be able to scan it fully and remove any malware that is installed, or the laptop will shut down before the scan and repair can finish.