The good news was that it was a math class--Algebra 1. I can do algebra 1 (for that matter, I can do geometry, algebra 2, and I know enough math analysis and calculus to get along for a day).
So, I did the only thing I could do--I found out where they were and I taught the next section in the textbook.
Luckily I had a cooperative group of middle schoolers. (A group of high schoolers likely wouldn't have let me take control of the class.) They went along with the lesson. They even did the work that I assigned. So, score one for me!
The teacher is out with some mysterious illness. No one in the office knows anything. I'll be back tomorrow, but at least I have a plan for tomorrow. And next week is spring break, so hopefully the teacher will be back then.