One of my users all of sudden cant sign in. whats going on?

Thank you for dropping by here in the Community, @kilowhatt. Allow me to share some insights about your query.

When a user encounters problems with QBO that appear to be browser-related, we recommend that they clean their cache. Temporary internet files increase loading performance, but too much can build up and cause operations to slow down.

To perform the browser troubleshooting. These are the steps:

1. Restart your browser.
2. Use other browser like Incognito.
3. Clear your browser’s cache and browsing history.

You might also want to give this article a visit, it’s about fixing common sign-in errors in QuickBooks Online.

If the issue persists, can you provide the exact error message?

Let me know how it goes by leaving a comment down below. I’ll keep an eye for your response. Assisting you is my top priority. Keep safe!