First thing to try, assuming that the website itself is responsive and not overloaded, is restart your phone before trying again.
If the website seems slow, just wait and try again. If your download gets interrupted because the site is overloaded, the jailbreak will fail.
If both of those fail, or aren't a problem, try restoring again. I've had the occasional weird problem where a restore doesn't seem to "stick." I have no idea why, because they should, but sometimes, when all else has failed, another DFU restore does the trick.
For future reference, restoring instead of upgrading is best practice. It may take a little longer, but it's the more reliable way to get back to a 'known good' condition for your device.
Good Luck!