If you are using a correct URL (check you typing, LOL), I can only assume that there is a problem somewhere in the network between your ISP and National's ISP. Hope it clears up soon.
FWIW, this is what I use to get to National's website:
www.nationalcar.com
It redirects me to the login/rate quote page.
poohbear2 - I must admist I am having the same problem, LOL! My rate went from $118 for 8 days to $242 for 8 days - same codes! Not sure what's going on with that - we aren't going until December!!!
Best wishes-