What to do if I was wrongfully accused and detained for shoplifting?

Well, that's hard to say. But I will tell you that you won't be able to successfully sue either of them. If you were at the store during the time that the rings came up missing, and you looked at the rings in question, then they very likely had probable cause to detain and question you. I know that spending two and a half hours dealing with this was inconvenient and probably embarring, but be glad that they finally realized that you were not the culprit. But they do have the right to detain and question possible suspects. I am very sorry that you had to go through that. But again, at least you didn't have to hire an attorney to defend against bogus charges. Good luck, and congrats on your engagement.