I would say, don't panic. See what happens tonight. I know when they regress it seems as though they have forgotten everything so its as bad as night one. However, the key to the sleep training is consistency. So whatever you do, don't change anything tonight.
There could be other explanations - more teeth, cold, tummy bug, just a bad day, a bad dream. All of those will make themselves apparent in the next day or so apart from the bad dream which hopefully won't happen again tonight.
You haven't done anything wrong
Let us know how you go tonight.