Sprint tried rolling out a website upgrade/overhaul on the weekend, but have suffered outages since Sunday because of it. The main site appears to be up and running again, but the “My Sprint”, “Support”, and “Community” sections still appear to be broken.
Sprint have said that they are allowing some traffic through, and monitoring stability for those that are allowed in.
In the mean time, if you need to access their account services, Sprint suggest either dialling (for free)
You can get updates about the site availability from their Twitter account.