Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Lets try two steps, and if you see any error messages on screen with the second step let me know.
Shut down your Mac.Locate the following keys on the keyboard: Command (⌘), Option, P, and R.Turn on your Mac.Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.Hold these keys until the computer restarts and you hear the startup sound for a second time.Release the keys.
Then, immediately shut down the mac.. And then we want to start in single user mode and run the command on the following page 2-3 times..
Once thats complete, before you exit single user mode, disconnect any USB drives, SD cards, phones, external hard drives, and remove any cds that might be in the tray.
Then, exit single user mode and try to start normally.. What exactly happens?