Why is Logic seizing?

I'm using Logic Pro X and suddenly every time I record a software track, the song and playhead plays the first note and immediately stops (even if it's a really small file, practically no tracks). If I hit the space bar or click the play icon, the track starts up for a fraction of a second but immediately stops again. This same thing occurs if I try to use automation at all, and has also happened a few other times for reasons I can't pinpoint (which I can only "fix" by undoing the last few things I've done until it "unsticks").