I have the GZ-HM320. I recorded a pretty dark concert of my friends band. I stayed in the same spot the whole time, but the camera lost focus sometimes as the singer moved. What do I do?

This is typical with low light and autofocus. You should switch to manual focus in such low light situations.
Joachim

What do I do?

If you stay in the same spot; zoom in completely, focus manually and keep it manual and don't touch teh focus again, then you can zoom in and out without loosing focus, even if they turn off the lights.