So maybe I'm the bigger idiot for using Google's site analytics on an important site but I did. This morning revenue was down so I loaded up my site and ... well it took about 5 minutes to load.
It turns out the Google Analytics domain which is in my tracking script (google-analytics.com) can't be loaded from my ISP. Given that this code is in the header of all my pages it managed to stop every page from loading.
I'm not sure but this seems to be the case from a few other servers too:
Pinging google-analytics.com [18.104.22.168]:
Ping #1: * [No response]
Ping #2: * [No response]
Ping #3: * [No response]
Ping #4: * [No response]
Done pinging google-analytics.com!