I'd like to know first if you encountered an error while trying to login. We need to make sure that we entered the correct credentials. If you forgot your User IS or password, you can reset it by clicking the I forgot my user ID or Password.
Also, if you have other browsers, you can use them so you can get into your account. Aside from that. let's make sure that you're in the correct login page. For QuickBooks Online, make sure to use this link: https://qbo.intuit.com/login.