Can anyone please help me to reboot my PC?

My pc restarted unexpectedly for no apparent reason. This has happened two or three times before and a reboot sorted out the problem.
Yesterday the screen froze and as I was unable to "escape", I tried to reset the computer with no success.

I have switched off the power, tried the reboot button, but to no avail. On start up, I get a windows message with options to start in safe mode or normally etc. It indicates that windows will start in 30 seconds, but nothing happens after the count down.

Your assistance will be much appreciated.