Green bar prevents playback...

The band and I were recording when suddenly we couldn’t get our tracks to play. The bar indicating where you are in the track had changed to green and whenever we tried to press play it just sat there. What happened?

First make sure that you still have plenty of free disk space.
Try clicking the “Fit Project” zoom button so that the whole project is visible.
If that doesn’t free it up, try rebooting.