At the risk of being obvious, could you verify that the file is in the proper directory and has not lost read permissions and the containing directory is readable (and has execute permissions if you are on osx or Linux)?
If you are on osx or Linux could you do a process trace to see which file it is accessing and the success? That is, to track whether the problem is in serving the file or in transfering the file, or in the target deciding the file is not valid
