If we knew what the keyboard was attached to, it might help. Several years ago I had several keys come off (my fault) on my 2010 MBP. I never could get them back on, and discovered that they were broken just enough so that they would never reattach. There were sources for replacement keys all over the internet, but the prices were very high. In the end the cheapest repair was for me to buy a replacement keyboard and put it in myself. In you case if you just need one key, I'd look at a just buying a new one online. Avoid the CA adhesive for a fix.