I put Pokemon Alpha Sapphire in to test it out, and it loaded up just fine, which is why I think it's the cartridge. I think it's something to do with the age, or that it's been gathering dust inside the DS for the past few months.
So, I don't know if this is the right place to post this, but I'm honestly so confused about how or why this happened. I've owned the same physical copy of New Leaf since release day. I haven't played it in the past few months, but it's been in my 3DS since it's the only game I really play...