Stuck behind a locked door in Sector 3 of Metroid: Other M? Fret not, Nintendo's got your back with the handy-dandy mail-in-your-saves solution!
The problem appears to be a door that remains locked in Sector 3. Nintendo of America explained in a reply to Kotaku that the issue is very much avoidable, albeit strange. The problem arises when players decide to backtrack before proceeding through the door.
However, if it's too late for you and your save file, Nintendo is also offering to fix the problem, free of charge. Basically, you'll have to send in your save file via SD card. There are other options available too.
Instructions can be found on the official Nintendo website.