MacBook showing 'Stop sign' on startup

Hey everyone,

I'm looking for some help if possible :)

(Sorry if this is the wrong place to post this, please feel free to point me in the right direction if it is).

So I have a late 2011 MacBook Pro running El Capitan.  Yesterday I decided to install Windows on it via Boot Camp.  So I loaded up the Boot Camp installer through the Mac OS.

I created a partition and then inserted the Windows 8.1 CD and clicked to start the install process via Boot Camp install assistant.

The Mac restarted and loaded up the windows installer.  I installed Windows and everything appeared to be fine (in as much as I was using Windows for the rest of the day once it had installed).

This morning I turn the MacBook on to use the Mac OS as usual, but before the Apple logo appeared I was immediately presented with a 'Stop sign.'

I have attempted a couple of things to remedy this:

• Tried booting in safe mode holding shift key on start up.  Didn’t work.
• Tried resetting PRAM holding CMD+option+P+R.  Didn’t work.

I then decided to load into Disk Utility using CMD+option+R:

• The Macintosh HD is greyed out and it says the Macintosh HD is not mounted.
• I select Macintosh HD then click ‘Unlock’
• I can then 'Verify Disk', it says ‘The volume Macintosh HD appears to be OK.’
• I then ‘Repair Disk’, says ‘The volume Macintosh HD appears to be OK.’
• Restarted, still get the stop sign :(
• When I go back into Disk Utility, the Macintosh HD is greyed out again and saying not mounted.
• So I unlock the disk again.  I can still verify disk and repair disk and it says it's OK.
• 'Verify disk permissions' and 'Repair disk permissions are greyed out'.  It says I can’t click these because the selected disk is not a start up disk.
• When I select the '750.16GB TOSHIBA' drive in the sidebar and click Verify disk, I get an alert that says ‘Storage system verify or repair failed.’  It says this when I click Repair disk as well.

I don't suppose anyone has any thoughts on this do they?

I know the likely answer is probably to format and re-install the Mac OS - in this case, does anyone have any support for how I can install Windows without messing up my Mac HD?

Thank you all very much in advance for any help! It will be greatly appreciated :)

So it seems like the answer to this question was to either boot into recovery mode with CMD+R and install the OS again with the chance to save your data. This however wasn’t an option for me because it kept trying to boot into internet recovery mode, and that told me I couldn’t install the OS because a newer OS was already installed.

So the option then was to create a bootable USB installer, but El Capitan is extremely hard to hunt down these because the other Mac I was using to create the bootable USB had Mojave installed, and that OS does not let you download El Capitan for some reason. (Apple restricts it, not sure why).

I basically bit the bullet and just erased the HD, then performed a fresh install of Lion, then upgraded it to El Capitan.

I just wanted to say thank you so very much for all your help and support! Really do appreciate people taking time out of their day to help me :)

I wish I had just done that from the start rather than wasting time trying to install the OS without erasing first and creating a USB installer haha.  It just seemed like it should have been a really simple thing to do, that turned out to be weirdly complicated by the obstacle of El Capitan being really hard to hunt down!

Thanks again anyways everyone :)