That could be a host problem. A little bit of downtime is pretty common but I'd contact the host to see if they are having server problems.
Or it might be that the file is too large. Are you optimising all your graphics. You want to aim for a file size of about 25k - less if possible. Have you added any photos recently without compressing them?
Another thing might be uploading the file into the wrong folder so as far as you know you have definately uploaded it but it's sitting in the wrong area of your site. I've done that with subdomains.