I'd like to help you access QuickBooks. However, I need more details regarding the problem. What error message did you encounter when accessing QuickBooks?
Additional details such as screenshots can help me come up with a resolution.
In the meantime, you'll want to do the following steps to help you access your QuickBooks Online account.
Regardless if there's a specific error message or not, a cache issue can prevent you from accessing your QuickBooks account. In this case, you can confirm this by using an incognito window.
This is a good way to rule out any cache-related problems. You can use these shortcut keys to access the incognito window:
Google Chrome: Ctrl + Shift + N
Firefox: Ctrl + Shift + P
Safari (new versions): Command + Shift + N
Next, log in to QuickBooks as you normally would. When you're able to do so successfully, close out the private window and clear the cache from the regular browser. Close it (all instances) out to apply the changes to the cache storage.