Particle.h not found?

OK, I have some code that has been working/compiling for months, and this morning it fails to compile…

In file included from lib/DS18B20/src/DS18B20.cpp:1:0:
lib/DS18B20/src/DS18B20.h:3:22: fatal error: Particle.h: No such file or directory
#include <Particle.h>
compilation terminated.
make[1]: *** […/build/target/user/platform-6lib/DS18B20/src/DS18B20.o] Error 1
make: *** [user] Error

Make sure you’re using a recent version of system firmware?

Did you forget to set a device target?