Q: I can't download an app using itunes on PC

Hi it's my first time to use an apple product. I'm using iphone 5s ios 9.2.1

The thing is I can't download an app using itunes on windows PC. I can download before but now I can't I don't know what happened.

Every time I try to download an app I click download -> itunes will access to itunes store (message appears) -> then it stops working. (Usually I can see the downloading notification above it but now nothing happens)

-Download button gets blurred.

-No current downloads appearing.

My internet connection has no problem.

I wish you guys can help me thank you.