Prohibitory sign on startup of mac

If when you start up your mac, you receive a prohibitory sign (a sign that is a circle with a slash through it–for some reason, i think of it as a do not enter sign, even though it doesn’t look at all like one) try starting it in safe mode. To do so, hold down the shift key at startup. If it boots, you can try either removing the automatic login items, or just restart from here. Sometimes just the restarting after safe mode fixes things. I’m not sure why. But I heard it from a guy at Apple, and found it to be the case.