Why am I unable to upload pictures?

I am using the same computer I have uploaded pics onto before. I can't upload pictures on RIU. The manage attachments pops up, I fill in the fields with the pictures I want, and when I click "upload" it only takes a split second before it says "done" in the bottom left hand corner. But nothing has been uploaded.
I am having the same problem with my yahoo account, and on any webpage. Can you help me? I need to attach files to an email for work... and upload pics on RIU of course!

I am able to upload pics again. I had to restart my computer. Any idea why?