Machine got hanged and displaying this message :“You need to restart your computer. Hold down the Restart button for several second”
This could be,
1) Bad Hardware Issue.
2) OS Issue.
3) Some serious incompatibility Issue with a driver or kernel extension.
4) Disk Permission Issue.
5) Corrupted system files or corrupted HD format.
How to Reset PMU (Resetting PRAM & NVRAM)
a. Shutdown your Computer.
b. Turn on the Computer and Hold the buttons “ Command”,”Option”,”P “and “R”.
c. Hold the keys down until the computer restarts and you hear the startup sound for the second time.
d. Release the Keys.
If this solution cant solve your issue ,then you need to reinstall OS