The last time this happened, it just started marking the songs missing (they got a little (!) symbol). I restarted iTunes, and it complained of the library not

The last time this happened, it just started marking the songs missing (they got a little (!) symbol). I restarted iTunes, and it complained of the library not being found. Fixed the mount, fired it back up, and played the missing songs again. You have to try playing those again if you want the songs to come up in shuffle, since it's the only way to clear the "missing" status.