Blank screen when logged in

I have ubuntu 16.04. several times it had asked for my permission to download updates but I denied.

Last time, when i shut down the computer something poped up, like some internal updates completed 66%. I had no knowledge of any ongoing process in the background when i clicked logout button. And there was no internet connection so i have no idea what kind of updates it was doing.

Now, when i tried to log in again, my computer started as usual.i entered the password and it suddenly showed broken logo of ubuntu. Then its just blank screen. I tried to login several times thereafter..and it showedd when i entered wrong password,but after that nothing. Only a blank scrreen wih moving cursor.

My question:

1. How to fix this or ugrade to new version of ubuntu?
2. How to get back the folders in my computer so that i can copy them somewhere else?

Help please.