Obviously you can't lose significant muscle mass in a day, or even a week. You could in a month if you got really sick and basically didn't eat. But honestly, it takes a while for muscle to go away, and for that to happen, your diet needs to be complete shit, or you need to be ill. Proper hydration contributes significantly to strength on a day to day basis significantly, so if your salt intake happened to be way too low that day, or you just didn't drink enough, you could have felt fine but been weak in the gym.
As for your overall training, I don't think hitting failure or close to it is a problem for the smaller lifts, particularly in higher rep ranges. I'll do things like curls to failure in the 10-15 rep range on a regular basis. What you DON'T want to do is hit heavy compound movements in lower rep ranges regularly.
Anyway, my overall opinion is that the first paragraph I wrote is more applicable than the need to alter your training. Chalk it up to a bad day, and keep working!