Reinstalling OS X on Your MacBook
This question seems to get a lot of attention on Mac-related Internet discussion boards and Usenet newsgroups — and the answer is a definitive perhaps. (That’s really helpful.)
Here’s the explanation. You shouldn’t lose a single byte of data by reinstalling OS X, so it’s definitely okay to try it. However, reinstalling OS X isn’t a universal balm that fixes all hardware and software errors. If your MacBook has contracted a virus, or a system file installed by an application “goes rogue” from data corruption, reinstalling OS X likely won’t solve the problem, and you’ll be back to Square One.
Follow the MacBook Troubleshooting Tree all the way to the end before you consider reinstalling El Capitan. Contact an Apple support technician via the Apple website before you take this step.
To reinstall, you can turn once again to the El Capitan Recovery HD volume. (That thing really comes in handy!) Hold down the cmd+R key shortcut immediately after you hear your MacBook’s startup chord to boot directly to the Recovery HD partition. Click the Reinstall Mac OS X option in the window that appears.