You would do the command-s boot in order to force the boot volume's file system, and repair if necessary.
At the prompt (after all the text stops showing up) you type /sbin/fsck -fy (thats fsck single space dash fy) return '''Note that this may not be able to fix all problems in a single pass,''' so ''if it finds and fixes anything (it'll print "***** FILE SYSTEM WAS MODIFIED *****"), run it again, and keep running it until it comes back with something like "The volume Macintosh HD appears to be ok."''
If this doesn't work, or you could try to run disk repair (command-D) boot to get to disk tools to check the volume, or repair the OS X system.
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You would do the command-s boot in order to force the boot volume's file system, and repair if necessary.
At the prompt (after all the text stops showing up) you type /sbin/fsck -fy (thats fsck single space dash fy) return '''Note that this may not be able to fix all problems in a single pass,''' so ''if it finds and fixes anything (it'll print "***** FILE SYSTEM WAS MODIFIED *****"), run it again, and keep running it until it comes back with something like "The volume Macintosh HD appears to be ok."''
If this doesn't work, or you could try to run disk repair (command-D) boot to get to disk tools to check the volume, or repair the OS X system.