Can't open or upload files from finder now I've updated to Yosemite
I updated to Yosemite over the weekend. Since then some of my files can't be clicked on to be opened, or to be uploaded online (e.g. hotmail online, or to a commercial printers website). I'm using dropbox to store most of my stuff on. However, some files can be accessed (dropbox and non-dropbox) and I can't work out any pattern as to why some files are accessible and some aren't. I'm totally frustrated, and would be most grateful of any suggestions/help.
MacBook Pro (13-inch Early 2011), OS X Yosemite (10.10.1)