Shutdown your Mac.
Restart and, as soon as you hear the startup chime, press and hold the “OPTION” (or “Alt”) key at boot until the boot menu appears.
Then, select the “Recovery" volume.
(Alternative: If the above doesn’t work… Restart, hold COMMAND + R keys until you see the Apple logo and progress indicator. If neither of those work, try COMMAND + R + OPT.)
Wait for “OS X Utilities” menu to appear (DO NOT CHOOSE TO INSTALL)
Choose Disk Utility from the available options.
In Disk Utility, select your normal, bootable hard drive volume from the list in the sidebar (usually "Macintosh HD").
Then, Click "Repair Disk"
If it finds errors, repeat the process until it reports the drive is OK.
Next, click "Repair Disk Permissions"
Once the process has completed, choose “Disk Utility” > “Quit Disk Utility” in the menu bar at the top of the display.
Then, restart your Mac and it should boot back into your normal Desktop.