An issue with CNAMES, create a support ticket ASAP.
As I received a similar support case recently, I thought I'd post with some quick troubleshooting steps anyone can try.
1) Does the CNAME resolve outside your company network? Try accessing the page from outside your company network (via your cell phone disconnected from wifi, or take your laptop to a coffeeshop). If you can access it from outside your company network, then it may be an issue with your internal DNS configuration and you should contact your IT staff to find out why it's not resolving internally.
2) Try using a free DNS tool like MX toolbox to see where the CNAME is pointing:
it should return accountstring.mktoweb.com for the "Canonical Name", where "accountstring" is the Account String listed in Admin > Landing Pages.
3) If it doesn't work outside your network, and the it appears to be pointing to the correct location, contact Support.