is there a way to reverse whatever happened to my phone today?
It does look like it did a factory reset, I'm unsure why, but that appears to be the case. Did it get wet? or very cold?

Unfortunately, there is no way to recover all your data (unless you took a full backup onto your PC). However, as you probably know, many apps sync with the cloud, so it isn't that much of an issue.

If this happens again I would recommend contacting Samsung (if your phone is still under warranty).