Reboot and retry, this is a sign of lack of resources or a rogue plugin often enough.
Rogue plugin may be having trouble writing a track that is not consolidated and stalls.. or some other weird thing. WHat plugins are in use?

Got enough disk space and RAM?

What soundcard and OS?

This is always valuable to know when trying to help.

May also want to try saving as a different name and see if it will then render from the new copy.

I do not work here, but the benefits are still awesome
Make your sound your own!