2009 Macbook Pro - Black Screen Boot
Yosemite installed fine, closed laptop for the night. Next morning, black screen but computer appears to be at login screen. Pressing power button shows garbled login screen momentarily then goes back to black. I've heard that Yosemite has uncovered hardware flaws in older macs. I'm trying to revert to Mavericks but booting to USB doesn't appear to be working. Any help is greatly appreciated.
MacBook Pro, OS X Yosemite (10.10.5)