I accidentally deleted the whole thread instead of just a particular message; surely there's a way to get all the thread back? I need this asap for a legal matter.
If that's from a Samsung phone and you have a Samsung account where your phone backs up messages, you might get it back. I'm not sure if your phone isn't a Samsung.
I did that on an iPhone. There is software, which may work on an Android, that will allow you to see, though not necessarily restore, the deleted conversation, but once you see it displayed on a computer screen, a record can be made at that point. The software has to be used before that area of storage is overwritten.