I agree. Don't where you're located, but this time of year in most of the U.S. is prime time for outdoor containerized plants to require extra due diligence as far as watering. Although I do check each pot individually every day, unless we get a rainstorm, I find that this time of year in Virginia my bonsai require almost daily watering.
All may not be lost with your elm. While it may end up losing all of its current leaves, if you caught the watering issue in time (& if that WAS the reason) & correct it, it may very well spring back with a fresh leaf crop. However, this may take quite a while, so you'll need to be patient.
My body is a temple. Unfortunately, it's a fixer-upper.