Hi
At the moment your best shot is tho get a upclose look at the board around where the B button sits and look for a broken trace and then repair it, i would also keep a eye out for loose connections on the bigger chips with the metal legs sticking out of them,to test the connections get a thin tool and very very very carefully tap the side of all the individual legs to make sure they are solid (if they are loose they will move and you need to solder them back down or run a jumper wire to the leg if necessary)
I would use this video as a guide on spotting broken traces and how to fix them
[link|https://m.youtube.com/watch?v=ref9JHUf-uw|https://m.youtube.com/watch?v=ref9JHUf-uw|new_window=true]
Just as a heads up: this sort of repair isn’t the easiest for beginners in soldering so if you aren’t comfortable doing the repair I would take it to a repair shop that’s reputable to have it done
Let me know if you can’t find a broken trace or a loose connection
Hopefully this helps
Any questions please ask
Thanks:-)