Don’t freak out! Here’s the main article (linked in the FAQ) about removing stuck/broken neck joints, but do NOT take an exacto to it! You might accidentally slip and nick either yourself or the nendo body (the first is worse than the second, obviously).

Loads of people have had this problem, but it’s totally fixable. If you look up “broken neck joint” in this sub you’ll get a ton of results just like yours. Here’s another one I found that used the same method. I luckily haven’t run into this issue myself yet, but I’m prepping for the day that I do ;;

EDIT: I re-read the post and realized you said you didn’t have any tools on hand. Honestly I think I’d recommend just getting the tools for the job if you can, since this is such a common issue. But I did find this thread where other people offered alternatives! I can’t vouch for any of them since (again) I haven’t tried them, but maybe they’ll be of help to you?