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What model Mac is that?
1. Shut down the computer. 2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected. 3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time. 4. Release all the keys and the power button at the same time. 5. Press the power button to turn on the computer. 6. Press and hold the Command-Option-P-R keys before the gray screen appears. 7. Hold the keys down until the computer restarts again at least twice. 8. Release the keys and immediately hold down the ‘Command’ + R’ keys on the keyboard during the start up process. If you can't get to that menu just hold the power button for 10 seconds until it shuts down. Then start it up again holding those keys. When you see the Apple Logo you can let go.
When the machine boots you may have to choose your language then you will see the OS X Utilities screen. Select the Disk Utility and then Continue. Then in Disk Utility select Macintosh HD in the left column (if there are two with that name or it is named something else choose the first indented item) and then on the right select the First Aid tab then select Run.
OK but I prefer you do these things in the order so I know for sure... Then your only option if that is ok is to reboot to Recovery HD erase your drive and start over from scratch. Without a backup of course you lose everything. Let me know...
You can try to save your files using Target Disk mode with another Mac if that is an issue.
This is Target Disk Mode you can try and save what you can before you erase and reinstall: target disk mode
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