This is a common problem. All you need to do is shut the computer off and plug it in to the wall. After you do that you need to hold down the left shift, option and control and the power button for 10 seconds. It should not turn on. After that you need to just turn on the computer as normal. This should fix your problems!
Did you try the technique I showed you? If you did it and it did not help you, than you should try seeing if startup programs are the problem.