I need a few things to make a diagnosis.
1. What type of file is it?
2. How large is the file?
3. What browswer are you using?
4. Where are you uploading from (edentube, the review page, media library)
The first thing you should always do is clear your cache and try again. Also, try to avoid Firefox browser when you're having problems. I don't know why but sometimes using Firefox makes them worse.