Audio stops making it to master

For some reason sometimes reaper just stops sending audio to the master. I have no idea why. There is no audio playback and there is no VU level, but the tracks are all showing output,just not the master. Some of the busses also stopped too. I found a workaround to keep the project alive ( by adding a hardware output to each bus) but now I can't see my output VU. What happened? How do I reverse/fix the problem

Are saying that is was coming throught the master and the suddenly stops? I sometimes find when I load a project, I can see individual track meters but no master meter and no sound. I close reaper and reopen and all is good . I have no clue why this is.

sometimes my busses randomly die and i can't fix them. just have to recreate them from scratch. fortunately it's not often. but usually if i have a problem with master output it's that my card sample rate got switched somehow and doesn't match reaper anymore (soundcard configuration problem). or like reno said and it's a volume spike, but then you'd notice the mute button on the master...