Dear Dana,
Thanks a lot for sharing your situation here.
Here are some things you can try:
1) How many labels are your trying to print? Sometimes when trying to generate a lot of labels (5,000 labels or lots of labels with pictures), Google Apps Script might have trouble handling the volume. The solution in that case is to split your mailing list or to filter your spreadsheet to print your labels in smaller batches.
2) Can you try refreshing your page, log out from your Google account, quit your browser and then log back in? It might help solving a connection issue.
3) If that still doesn't work, can you try printing your labels in private window?
Keep us posted,
Lucy - Labelmaker