Macbook Pro won't boot past grey screen with Apple logo and then shuts off
This is a long story...
This morning, I would turn it on and it'd go to the grey screen with the Apple logo for a few seconds and then go to the icon that is a circle with a line through it. After reading online, I was able to boot it up in Safe Mode. I messed around a little bit in system preferences thinking I could fix it on my own, not really knowing what I was doing (I'm a *******). I went to Startup Disk in Preferences. I chose the MAC OS X, xxxx and then clicked the lock to prevent further changes and then clicked restart.
Now when I turn on the macbook, it goes to the grey screen with the Apple logo for at least two-three minutes and then just shuts off. Won't boot into Safe Mode now. Please help. I already tried the Command-Option-P-R keys trick like five times.
MacBook Pro