I've provided a few troubleshooting steps for the possible reasons why you're having a login issue.
Let's verify if this is a browser-related concern.
A cache issue might be causing a browser's unusual behavior. You can use a private window and check if you're able to log in there. A private window mode is an Internet browser setting that prevents browsing activities from being stored. If you do, go back to the original browser and clear the cache. This way, the internet files that might be causing the browser's unusual behavior will be deleted.
Also, I'd suggest logging in using a different browser such as Google Chrome, Internet Explorer and Mozilla Firefox.
Thus, if this is a login credential issue, please make sure that your Intuit credential is connected to your email. This way, you'll be notified once you'll have to make changes with your QBO account. In connection with this, you can click the I forgot the user ID or Password hyperlink in the Sign in page. An email will be sent to your email about resetting your password.
Please let me know how it goes. I'd be right here to help anytime.