You can try to spot errors with Event Viewer
Open the Start Menu
, and copy eventvwr.msc
in the search box and press enter.
If prompted by UAC, then click on Yes.
In the left pane of Event Viewer, double click on Windows Logs to expand it, then right click on System and check for errors (see screenshot). If no related errors will be in System, please check Application section.
Please post description of errors occurred within the described time of sudden shutdown.