Hi, welcome to justanswer.com! My name is Mike.I am going over your question and I will be with you shortly.
Is that an iMac or Laptop about what year?
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, if it starts up you held the power button before the other keys, try again.
4. Release all the keys and the power but
ton 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. (You have to do it pretty fast if you miss it just shut down by holding the power button for 10 seconds then start it up and continue with step 6).
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. 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. Let me know the results....
Hold the power button for 10 seconds then restart and hold Option Command R, this will boot to internet recovery and then follow the previous instructions, let me know
It will take some time it has to boot from Apple's servers so let it go for a while....
At some point you should see a globe if you don't then you may not have hit the keys at the right time, try again.
Do you still need help you never replied back?