Hi @kroutt1 
Welcome to the Canvas Community. Thanks for posting your question. I am sorry to read that you accidentally deleted a student's discussion posting. I'm not quite sure that this will work, but you can give it a try: Undeleting things in Canvas. If this trick doesn't work, then I would recommend that you reach out to Canvas Support...as they might be able to retrieve the posting for you. How do I get help with Canvas as an instructor? Good luck, Katrina!
