A server connection, while not required to access all of the game's content, is necessary for handling virtual currency – which players use to purchase goods and attributes for their custom players – as well as certain modes such as MyTeam and NBA Today. When the game fails to connect to the 2K servers, it locks players out of this content.
An update, issued for the game last night, seems to have resolved a majority of the connection issues, though there are still problems with MyTeam and private TeamUp matches. 2K Games' Ronnie Singh has been active on Twitter today, providing status updates as the developers attempt to quell the server issues. Current-generation versions of the game seem to be good to go, while next-gen versions should have MyTeam working again in the "next few hours."