Try these first.
http://support.apple.com/kb/HT1379?viewlocale=en_US&...
http://support.apple.com/kb/HT3964
If they fail try starting with command and R pressed to bring up the recovery partition. Try didk utilities first. Then check the startup disk and make sure it is set correctly. If all that fails you'll need to consider attempting a reinstallation from the recovery partition.
Des
Sky Broadband, Wired, Wireless, VoIP, 1 Mac, 2. Hackintoshes, 1 PC, 2 HTPCs, iPhone, iPad, OS X, Windows 7, Hate and 8 rhyming is not an accident!
Rehab is for quitters