try booting with the "S" key held down
iMac running snow leopard (or mountain lion whichever is more recent), won't get past the white screen with apple logo, gray spinwheel, and load bar at bottom. Bar gets to within a sliver then screen goes black and nothing. Is there a keystroke combination we can try during boot up? Already tried the zap pram trick (control+option+p+r) to no avail. Any ideas without getting overly technical?