Cannot open Keynote files after closing the cover

Product Name: Macbook Pro Retina 13' (Late 2013), ME864

Specs: Processor Speed: 2.4GHz Intel Core i5

Memory: 4GB 1600MHz DDR3

Storage Capacity: 128GB

Operating System: OS X Yosemite 10.10.2

Keynote Version: 6.5.2(2119)

Problem: The keynote file was quite normal before: backup automatically, all things are working just fine. But just now when I closed the cover of my macbook pro and reopened it, the file is broken. The thumbnail images are all fine, and nothing is apparently wrong. And then I opened it again, and it says "Cannot open file "xxx"".

Troubleshooting steps I've already tried: I've tried the iCloud backup, but I can't open it online, or download it and open it, or convert to ppt.

Please help😀