Hi Gillian, I don't know the reason. Maybe there are something wrong with the internet connection or with the skype's servers. 
I think God help me :) 

I would try start from this.

https://www.reddit.com/r/China/comments/33ivl5/is_skype_banned_in_china/

Or try google something like this "is skype banned in china"

Thanks you all for responding. But the problem is still exist. I search the issue on internet, many people meet the same issue as mine.