Hey,
As per my understanding of your problem your computer restarts automatically. This problem arises due to malfunctioning shell-extensions, corruption of the system files, incorrect power settings, and so on. Since your question lacks detailed information about your machine I’m hereby providing complete troubleshooting guide.

[B][U]Diagnose your Memory[/U][/B]

1. Press Windows Key + X.
2. Click Search.
3. Type “Memory” in the Search Box.
4. Click the Settings icon.
5. Open “Diagnose your computer’s memory problems”.
6. Click “Restart now and check for problems” button. The computer will be automatically restarted and it’ll conduct several memory tests.

[B][U]Run the System File Checker[/U][/B]

1. Insert Windows 8.1/ 8 installation disc.
2. Press Windows Key + X.
3. Click “Command Prompt (Admin)”.
4. Type the following command:
   sfc /scannow
5. Press ENTER.

[B][U]Reset Power Settings[/U][/B]

1. Press Windows Key + X.
2. Click Power Options.
3. Click “Change plan settings” link ahead of the selected active plan.
4. Click “Restore default settings for this plan” link.
5. Click “Save changes” button.
   For more troubleshooting guidelines please [URL=‘https://www.youtube.com/watch?v=T4rWDGVCxMU’]watch this video[/URL].