It could be that the audio driver failed and Reason fell back to another driver.
If you don't select audio driver manually in Reason Preferences, Reason will try to open the last manually selected driver each time it opens, and if that fails fall back to the other drivers in some order.
The native ASIO drivers for some audio interfaces allow mixing in the output of other apps like web and music players. Some do not.
If your native ASIO driver doesn't support mixing, and you also have ASIO4ALL installed, maybe you were using ASIO4ALL in Reason earlier. ASIO4ALL will usually provide ASIO with mixing for interfaces that have a non-mixing native ASIO driver.