Key Combination for Restart During Boot My Mac gets stuck during bootup quite frequently recently (no entry sign and flashing folder). Every time it happens, I have to press the power button to shut it down and press the power button again to turn it on.

Is there a software reboot key combination like Window's Control + Alt + Delete?
Although Apple publishes a list of OS X keyboard shortcuts for startup, there is not one for forcing a reboot.

However, your problem seems to be indicating a failing hard drive, so make sure that you create a full backup of your data (if you don't have one already) as soon as possible.

I would replace your drive manually (if your Mac has a user-serviceable hard drive) or take your Mac into an Apple Store for service.  In the long term, this should solve your issue.