Same thing happened to me at Bear Mountain 50 a couple years ago. Warm, but not super hot, and my body just stopped sweating around 25-28 miles in. I knew this wasn't a good sign so I dropped and got a ride back. I probably could have walked it in - felt good otherwise - but didn't want to leave my ride waiting or worrying about me.
If I didn't have a friend that was running her first 50-miler out there, I might have dropped...but I figured as long as she was out there, I would have to try to keep going. I (1) wanted to be a good example for her, and (2) had to wait for her either way. Unfortunately, she ended up getting pulled at mile 38 (she didn't make the time cut-off).
Scary stuff, to stop sweating...I hope my body acclimates, since summer is coming up!
I never figured out what caused it, but it was pretty freaky at the time...