Mortal Kombat 11 got a massive update last week, and unfortunately, it seems to be causing a ton of problems for Switch players. Most people reporting issues can't seem to connect to servers, which blocks them from a number of different modes. On top of that, players are also reporting a lot more crashes after the update.
Warner Bros. Interactive has sent out a generic email response to someone complaining of the issues. They say to power cycle your Switch, or delete your install of the game/DLC downloads and start fresh. Obviously that's not really a true fix, so we'll have to hang tight and hope that NetherRealm/Warner Bros. offers a more permanent and viable solution.