Quite a few users have reported that their systems have been more or less bricked following the latest 3DS upgrade. There’s a large thread on Nintendo’s technical forums about the problem, which results from an unexpected error during the update process. Those who encounter the issue have been unable to access the eShop or System Settings.
There is a fix, fortunately! When turning on the 3DS, hold down the “L”, “R”, and “A” buttons while also pressing the “Up” area on the d-pad. You’ll then be brought to a system update screen to complete the process.