Hello Ian Coots,
I'm sorry you're having this trouble, especially with a document you've been working on for months. The good news is that you shouldn't need to purchase Microsoft 365 again if you still have a valid Office 2019 license.
Before trying the reinstallation suggested earlier, you can try something simpler:
- Open Word > File > Account.
- Under Product Information, look for Switch License or Change License.
- If available, select it and sign in with the Microsoft account you originally used for Office 2019.
- If your Office 2019 license appears, select it and follow the activation prompts.
You can also check whether Office 2019 is linked to your account here: https://account.microsoft.com/services
If the license-switching option isn't available or Office 2019 doesn't appear, let us know what you see under Product Information. We can then help you work out the next step without unnecessarily removing your software.