Hello, ftbethard.
I can help you out and make sure you're able to sign in to your existing online account on a different computer.
It's possible that a different email address was used on the other computer. Make sure you're using the existing online account's email ID when logging in.
Also, you'll want to avoid using the autofill feature in the browser when logging in. Manually type in your email address for the existing online account.
If you're having issues logging in, you'll want to click the I forgot my user ID or Password link in this page: https://qbo.intuit.com/. Then, enter the needed information in the box.
After successfully logging in to your account, you might want to read our articles if you need help navigating the program. They have guides and instructions on how to do a specific task in QuickBooks Online. Select a topic here, and choose an article.
If you have other concerns you want me to address, please let me know. Add more details about it and I'll get back to you as soon as possible.