Welcome to the forum, @brianchilders1. For the project that’s no longer playing back any sounds, try choosing Play > Playback Template and re-applying the default playback template. Hopefully that will restore playback immediately.
That worked, but now the Garritan sounds don’t play. I’ve tried reloading them and they all default to the Halion Sounds.
You need to combine the two libraries in the same Playback template to be able to apply the two libraries with the Apply and Close button of the Page Templates dialogue. To make this you can edit a copy of one Factory PT and add the Endpoint Configuration/s of Garritan and set the Family and Instruments overrides, etc…
There are several video explaining this.
For example:
And the manual: