First you need to reboot while holding down the alt/option key then when the two driver things come up I prefer you connect your laptop to Ethernet since wifi gets very slow. Now choose the recovery driver and once you are there, connect your laptop to a power source and choose reinstall whatever software it says. Follow the steps there and in 2 hours your laptop should finish the reinstallation no files should be lost or anything since for me my laptop still had all the memory plus I learned this form the Geek Squad from Best Buy so if it works which it probably will, then thank them and notm me
Good luck and if you need more information check this out
https://discussions.apple.com/thread/5605446