Sony PCG-F520 - Manuel (Page 69)

Troubleshooting #'
Power Management
The operating system may become unstable if a lower power state such as
system hibernation is initiated and then changed before the operating system
completely enters the lower power state. To restore the computer to its
normal operating stability, close all open applications, press the power
button, and hold it for four seconds or more to completely shut down the
computer.