You must stop it restarting to find the cause.
Open System in Control Panel.
On the Advanced tab, under Startup and Recovery, click Settings.
Under System Failure, select the check boxes that correspond to the actions you want Windows to perform if a Stop error occurs:
Write an event to the system log specifies that event information will be recorded in the system log.
Send an administrative alert specifies that your system administrator will be notified.
Automatically reboot specifies that Windows will automatically restart your computer.
Then you can look in Event Viewer.