ok file efxaristo
apodo kai kato den katalaveno mporis na mou exigisis sta ellinika kai anilitika se parakalo?

Open terminal and type following:

sudo -s
nano /Library/Preferences/SystemConfiguration/com.apple.Boot.plist

then add -f under the kernal flags section
(adding this flag also solved problems I had restarting & shutting down the PC)

then ctrl+x
then y
then exit terminal