Mac Won't Boot. Help needed

I use top-notch MBPRO15 Late 2011 with the latest OSX and all the recent updates installed. I had booting problems before which I managed to solve through command+option+P+R. I'm not sure what exactly caused the problem, but it helped a couple of times. But starting from yesterday I'm failing to make it boot whatever I do with it.

So I'm having this:

1. I turn on the Mac
2. Chimes go as expected and the initial login screen appears
3. I put my credentials and booting progress bar starts up
4. After like 30-60 secs the progress bar disappears leaving me only with a grey screen.
5. Two options possible afterwards (i) the screen stays grey for as long as I wait or (ii) the mac reboots leading me to the initial login screen.

When try to go to the Troubleshooter with command+R the sequence is exactly the same. Instead of the Recovery mode I get a frozen grey screen.

If you need any more information to localise the problem please ask.

Solution 1:

You may find these links of some interest:

https://www.facebook.com/2011mbp
http://www.apple.com/support/macbookpro-videoissues

Solution 2:

To fix this try the following:

• boot to internet recovery mode by pressing cmdaltR immediately after the start-up chime. Depending on your internet connection the boot process may take several dozen minutes.

• open Disk Utility, choose your main volume, verify and repair it if necessary

  

• if you had to repair errors just repeat the step to recheck that all errors are gone
• reboot to your main drive
• if the problem persists boot to single user mode by pressing cmdS after the start-up chime to circumvent the lockout.
• after booting is done just enter 'exit' at the prompt to start to the common Aqua interface.
• download Onyx or Yosemite Cache Cleaner
• start Onyx

• clean system/user caches

  

• or start Yosemite Cache Cleaner

• clean system/user caches

  

  ** "System" is the name of the start volume in my VM. Your volume name may be different.

• restart your Mac