What's the issue specifically? Does it start to upload and give an error or does nothing happen when you try to upload?
Does the upload take longer than you are used to? If there is a very slow connection you may be timing out before the upload is able to complete.
I would suggest as a first step to clear your browser cache (I can tell you how if need be) and restart your computer. Odd issues sometimes clear up with these two steps.
If that fails, try uploading somewhere else, like Photobucket. If that works then you have a site-specific issue we may not be able to diagnose.