cant get into my self employed account

I am unable to log into my account, I keep getting a message to ‘Dowload the Quickbooks accounting app. Looks like you’ve changed your Quickbooks accounting plan, to start using it on this device download the Quickbooks accounting app.’ 
I definitely haven’t changed anything and need to get in to mark invoices and send new ones. What do o do here? It won’t let me login using a code which I always do.