Can not upload images via admin interface

I suddenly was unable to upload images the last few times I’ve tried to create a post. I’ve logged in from 2 different computers with both IE and Firefox. When I try, I create a new post, click the “Add an Image” next to the “Add Media” label and this brings up a pop-up window. There is normally a text field and a “Browse” button that I can use to upload images from my computer. What has started happening, is that the text field and button quickly vanish and are replaced with a “Choose files to upload” button that doesn’t work (clicking the button does nothing). There are no error messages on the screen indicating any kind of problem. Has anyone else run into this? Is there a resolution or better ways of uploading images? I noticed that before, it was automatically creating smaller versions of the images for me so I would like to continue using the interface rather than directly FTPing the images up to the server. Please let me know if anyone can assist me with this problem. Thanks!