try a quick fix: shut down either by holding power button a long time [some say 5 sec, i say 25] or pull the plug, with power pulled off altogether hold the power button a few seconds, plug in, hold power button down 10 seconds to be sure, then release, press normal. some will go ahead and start while holding the power button in, and you may need to repeat, because the O -Option- key must be pressed/hold while initial bootrom comes up.
is this an intel-platform mac?
at any rate, holding the O [alpha] key should bring up the boot choices menu. if only one choice, take it. if 2 choose the name of your main Disc volume
