2011 MBP won't boot past grey screen

I came to turn on my 15" 2011 MBP OSX10.9.5 today, which had been left in sleep mode for a few days. The battery was flat, so I connected to the power supply and left it for a couple of hours. I then turned it on (no longer connected to power) and it made the familiar bong and showed the apple logo; the screen then went grey, changed a few shades of grey and then stayed there, with a cursor on screen, but nothing else. I powered off and retried.

I then booted successfully into the bootcamp partition, did my work in Windows and then tried rebooting into the OSX partition, with the same outcome as before.

I booted to the recovery console and did a repair permissions and repair disk on the OSX partition; it didn't indicate any particular problems. Still didn't boot though. The OSX and Windows partitions are on the same physical hard disk, which is an SSD which I fitted myself about a year ago (no problems till now).

These symptoms seem to indicate that there is not a problem with the hardware, as Windows boots and runs just fine. I am surmising that the problem might lie with the OSX partition.

Any suggestions?
Also, I should confess at this point that the disk has not been backed up fully, so there is some data I would like to recover ideally.....

This does sound like a confusing situation...since you are able to boot into the Windows partition, able to boot into the recovery partition...but not able to boot into the OS X partition.

Since there does seem to be a possible power related issue (battery was allowed to go "flat")...I would say try doing an SMC Reset:

Intel-based Macs: Resetting the System Management Controller (SMC)

And if this doesn't do it...try an NVRAM/PRAM Reset (don't worry that the link says "Mavericks"...it's the same procedure with earlier OS's):

OS X Mavericks: Reset your computer

HTH,

- Nick

Thanks for the tips. I've tried both of these to no effect. The pram reset produced a double bong but it's the same sequence when the mbp tries to boot: grey screen, white screen with Apple logo, grey screen with cursor. There are various flickering shades of grey, but it stays that way until the screen dims (some sort of power saving feature?).

more info: when I put the ssd in, I put the existing hdd in the cd bay. This seemed a common approach and I assume it's not really connected to my current situation.

If I whip out the ssd and connect it to my Mac mini as an external drive, presumably I can recover files? However, the permissions have stopped me doing this in the past?

I could then reformat the system partition and reinstall Mavericks over the network and if that works, try to reinstall my time machine backup?

Any better ideas?