Faders, pan, solo, mute inactive

I use Pro Tools MP9 with an M-Audio Fast Track Ultra 8R interface on a Windows 7 SP1 64 bit computer, Quad Core i5-3350P processor, 8G RAM. All optimizations have been done following Avid website instructions. This computer is dedicated strictly as a studio computer--in other words, little to none in the way of non-Pro Tools related software. I've been using this setup for about a year now without an issue, but yesterday I was doing a mixdown. I finished one session, then closed it and switched to another. As I played the song, I wanted to turn down lead vocal, but the slider wouldn't move on the Mix Window. I tried other sliders, but nothing happened. I tried the pan knobs...nothing happened. The solo and mute buttons didn't work either, but the song was playing along just fine. I tried the controls on the Edit Window. Volume and pan would move there, but had no effect on the volume or pan of the sound on the track. Solo and mute buttons there did not work at all. And no automation was turned on to any of these tracks yet. I closed the session and opened another...same thing, etc., etc. Finally, I exited Pro Tools and restarted the computer. After that, everything worked fine. Any thoughts on why this happened and/or how it can be prevented in the future? (BTW, I tried to post a Sandra Report, but was told I'd have to buy the pay version to generate a report. It said the free version would not generate a report. Is this something new?)