I'm here to provide assistance in logging in to your account.
We need to check first if your browser is compatible with QuickBooks Online. Clearing the cache and clearing browsing history can fix this as well. Here's how:
Google Chrome: press Ctrl + Shift + N.
Mozilla Firefox: press Ctrl + Shift + P.
Internet Explorer: press Ctrl + Shift + P.
Safari: press Command + Shift + N
This workaround can help you, click I forgot my user ID or Password link on the sign-in page. Then reset your password.