Wipe and reinstall OS X El Capitan

Install El Capitan from Scratch

Backup if possible before continuing.

1. Restart the computer. Immediately after the chime hold down the Command and Rkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Reinstall OS X and click on the Continue button.

Dear kappy, I’ve followed your instructions and all went well until I reached the part of downloading the software to reinstall. Every time I sign into the App Store to download, it says “This item is temporarily unavailable. Try again later.” It’s been over a day of this now. The operating system is OS X El Capitan. Any suggestions? I’m desperate to be able to use my laptop again. Thanks.

Have you ever downloaded El Capitan using your Apple ID?

If you have access to another Mac, try signing into the App Store using your Apple ID. Check the App Store Purchases tab. If El Capitan is there, download it. If not, try using the link below to download it. You can make a bootable USB stick to install using this free program which will do all the work for you. Boot off of USB and install the OS.

Bootable USB Flash Drive – Diskmaker X oruse Apple’s method Create a bootable installer for OS

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra or find an older computer where you can download it using your Apple ID.

El Capitan

I believe your computer is too old for the internet recovery method. Instead:

1a) Reboot with your backup connected while holding down the option key.

1b) Select the recovery partition on your backup drive as the boot volume.

2-8) Same as Kappy's response.

Creating a second redundant backup, while optional, is always recommended if you want to play it safe.