iPhone can't find my music (again)

This has happened a few tmes before, and is getting annoying. This morning, after working fine yesterday and charging (off a wall socket, not a computer) overnight, my iPhone cannot find any of the music loaded onto it. iTunes shows the "No Music; You can download music from iTunes" screen (though my podcasts are still there. It's not that the files are gone: looking at the About screen shows only 1.9GB available, so clearly something is taking up all that space; but rather, the iPod app simply can't find the music it had no problem playing for me yesterday.

In the past, I've been able to synch and re-load all of the music back onto the iPhone, and I have no reason to think that won't work this time. But synching from scratch is a pain in the neck (takes an hour or so), and does me no good when, like today, I only find out about the problem once I've left my house and want to listen to music on my commute or at work. So, not the end of the world, but has anyone experienced this problem and knows how to fix it?