Updated iphone 6 to ios9 and phone is hung on the white screen with a loading bar, no amount of recovery mode is helping

Updated my Iphone 6 to the new ios 9 at which point my entire phone data got erased and i was unable to recover any of it. Luckily i had back up from a few months ago.

Now aftre 2 weeks of that disaster my phone again switched on with the connect to itunes icon and asked for update of software which after hours of trying finally got downloaded and updated. But after the restart my phone keeps getting stuck on the white screen with the loading bar which does not progress beyond 1/6 of completeion. I have tried the recovery mode steps and switching it on and off nothing seems to help. It keeps going back to this same screen and then just stays like that. What do I do??

Also if anyone could tell me why this is happening so as to avoid a repeat.