For troubleshooting purposes it is the best idea to boot into Safemode. In Mac you can boot into safemode by pressing and holding the shift key while the system is booting up. It will turn off all nonessential system-software nubbins in an effort to get a sick machine at least powered up.
It can come in handy when you’ve just installed some new piece of software and find that you can’t even start up the machine, or when one of your fonts is corrupted, or when something you’ve designated as a Login Item turns out to be gumming up the works. With this trick, you can at least turn on the computer so that you can uninstall the cranky program.The trick is to press the Shift key as the machine is starting up. Hold it down from the startup chime until you see the words “Safe Boot,” in red lettering, on the login screen.