Merge

So I am getting back into Blender after a long time, and not knowing much the first time around. I modeled a hand with an arm and then ended up fixing the hand later on. I figured instead of remaking the whole thing i would separate the hand fix it up then reattach the hand back on. After fixing up the hand i tried reattaching it and just wont I’ve tried what seems like everything to get it to work but it wont. Its driving me crazy and I’d like some help if anybody is willing. If you need the file just ask. Thank You!!!