If you have a wireless keyboard, you likely will need to use a wired/built-in one to perform the at boot commands!.A systematic manner to get a booting machine and then work down through Applications and User account issues.The process of elimination is designed to narrow down your problem and expose unknown ones, fix it all yourself.Only Step #17 and 20 are harmful (to your data) and marked. Later steps usually depend upon earlier steps being performed.The Steps are numbered, if instructed, then just jump to do that Step or run through them in a process of elimination.The Steps appear to work also in OS X Mavericks (10.9) at this time. This User Tip is designed for Mac's that are capable of running OS X Snow Leopard (10.6) to any Mac running OS X Lion (10.7) or Mountain Lion. Please read the disclaimers at the bottom. If it's too much for you, there are local PC/Mac software repair and data recovery services, for hardware repairs you should take it to a Apple Authorized Repair. It's designed for those who already have a good familiarity with how Mac's work, is almost all Apple support documented. If this User Tip appears to be quite involved, it's designed to very through and systematic process to achieve success.