First off, if it is overheating, the computer will not come back up immediately.
Secondly, right click on the My Computer/This PC icon in Windows Explorer and select Manage. (must be run with Admin rights)
Expand Event Viewer (you may also need to expand Windows Logs, varies by Windows version)
Select System
Look for Stop signs or exclamation points
Post here in detail (example below):
Level: Error
Source: Service Control Manager
Event ID: 7000
Double click the event to find out what is being referred to:
The Steam Client Service service failed to start due to the following error:
The service did not respond to the start or control request in a timely fashion.
