I had exactly the same experience (except that I didn't install supersu) with my Xperia M C1905 a few days ago, and have been struggling unsuccessfully to fix it. In my case the phone got stuck at the animated boot logo after I unlocked the bootloader, even though I used the official Sony method and followed the instructions to the letter, and it said it worked OK with no errors. After that I successfully flashed a CWM boot image, and can now boot into recovery, but normal boot is still stuck at the logo. I can access the phone with flashboot but not adb. I have the correct Windows 7 drivers installed.
In CWM I find (like the previous poster) that I cannot mount or format /data, so I assume that's the cause of the problem. Does this mean that the data partition has got corrupted? If so, how can I fix that?
Thanks for any help!
---------- Post added at 12:44 PM ---------- Previous post was at 12:42 PM ----------
Thanks, but as a newbie (at least to this phone) I'd appreciate more detailed guidance as to how to find the correct firmware (I'm on Android 4.3 stock, UK) and how to flash it safely.