If your ultrabook does not respond, and you cannot restart it.  Here is what you can do to fix that issue.

If your Aspire S5-391 stops responding completely, press the power button for 10 seconds. The system should shut down. Power the system back on to resume normal use. Use the following steps to to restart if the system does not shut down:
  1. Disconnect the AC adapter.
  2. Gently insert a paperclip in the battery reset Battery Reset Hole Icon hole on the right-hand side of your notebook.aspire ultrabook hard reset factory recovery