It's a 2012 cMBP with Samsung 840 pro SSD. The problem is I'm getting the no entry sign (for a split second the apple logo will flash up). The weird thing is if I turn it off and leave it 30 minutes it will then boot up. I've done a fresh install, checked for any loose connections and the SSD is showing as 100% healthy but still the problem persists. Anyone had a similar problem? or an ideas what the problem may be?