Regardless of the cause, I feel this (as in submitting your IP to be individually whitelisted) is a pretty pointless and inconvenient solution to the problem. Not everyone is gonna want to DM a webmaster about access issues; it's so much easier to close the page and go to a different site...
Was happening to me yesterday as well, but was worse on my phone. My internet is so bad I just assumed it was my internet but I was able to load other sites. ZD outages happen less often these days so I'm sure it'll work out.