I'm here to ensure you'll get past this issue so you can get back to working order, Indi.
Before anything else, did you receive any error messages when trying to save and send an invoice? This will help me identify the root cause of the issue and accurate resolution for you.
In the meantime, I recommend performing some troubleshooting steps to help isolate the issue.
Sometimes, numerous cached files can cause unusual browser behavior. Let's access QuickBooks Online through a private window.
Here are some key shortcuts for accessing a private window:
Mozilla Firefox: CTRL + Shift + P
Microsoft Edge: CTRL + Shift + N
Google Chrome: CTRL + Shift + N
Safari: Command + Shift + N
If you're finally able to save and email the invoice after this, the root cause of the issue must be from your regular browser. I suggest clearing your regular browser's cache.