I'm running a macbook pro 2012 15" with Yosemite. I have an issue where the mac becomes unresponsive in a couple of different scenarios
1) Left with the screen saver running for a while, it allows me to enter my password, then does nothing, it seems to be waiting for something, I can cancel and re-enter password, still no response, it also won't allow me to click on the reboot etc
2) System running, leave the mac alone for a while, return and its very slow and eventually just sits with the spinny wheel.
In both situations a forced re-boot gets it going again.
1) Left with the screen saver running for a while, it allows me to enter my password, then does nothing, it seems to be waiting for something, I can cancel and re-enter password, still no response, it also won't allow me to click on the reboot etc
2) System running, leave the mac alone for a while, return and its very slow and eventually just sits with the spinny wheel.
In both situations a forced re-boot gets it going again.