Double-click System. Three types of events are recorded: information, warnings, and errors. Click the top of the "Type" column to sort by type, and then scroll down and find the Errors section. The first few records may reveal more about the event that triggered the problem.
It is very helpful to note the time that an event happened. This helps you to move back to find the last recorded event that happened before the shutdown.
Double-clicking an event will produce a popup that shows you the date and time it was recorded, the source of the error, and an "Event ID." It's not likely that you'll be lucky enough to find an event that says, "The system stopped because..."
So the next best thing is to find out what the last event recorded was. Here it may be necessary to note the exact time for each error, warning, and notification. This can give you a picture of what was happening with the PC right before the event happened. If the problem has been recurrent, you should be able to go further back in the event log and find a similar sequence that happened just before the previous crash.
If so, go back and check the Application section. Maybe something was being recorded there (like maybe a notification to the user) just before the crash happened.
Then look in the Security section as well.
When you have your Event ID or error message, go to Microsoft's Events and Errors page and do a search to find more information.