It's my priority to help you log in to your QuickBooks successfully and get back on track.
I'd like to know if you received an error message while trying to access your account. Any further details are much appreciated so we can narrow down this concern.
When you encounter an unexpected behavior while working with QuickBooks Online, it'd be best to access your account on a private window. This way, we can isolate if the problem has something to do with the stored cache files on your browser.
Here's are the keyboard shortcuts that you can use to open a private window:
Google Chrome: press Ctrl + Shift + N.
Mozilla Firefox: press Ctrl + Shift + P.
Internet Explorer: press Ctrl + Shift + P.
Safari: press Command + Shift + N
If you can now log in with no issues,go back to your regular browser and clear the cache to delete those temporarily stored files and browsing history. The cache files can often be corrupted and may lead to odd issues like the one that you've experienced.