How do I sync my files properly all I see is a red triangle with an exclamation point on top of each file??

I have a MacBook Pro with Retina Display running OSX Yosemite version 10.10 on a 2.4 GHz processor with 16 GB RAM and Intel Iris 1536 MB. The red triangle comes up whenever I try to upload files and now is a permanent fixture on the files. I can still create new files yet I want to upload my work. How may I fix this issue. Also I would like back my old files I lost from a lapse of less than a month in my subscription.