To get back to the previous build do this. It worked for me.

ok so you are going to have to hard restart your comp. then let it boot. when it get to the screen where you have to enter your password to login to your comp. press and hold shift, while doing this click the power button on the bottom right of screen. then while still holding shift down click restart. This lets you boot into advanced options. Click, advanced options, then click rollback to previous build, then follow the steps from there.

Hope this works for you!

FULL info here :

https://www.reddit.com/r/Windows10/comments/4vufpo/windows_10_fully_freezes_after_anniversary_update/