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 CMD+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 CMD+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.