I don't believe it's a systemwide problem, since I've had no problems logging into my return multiple times today, and there are not (from what I can see) other users asking the same thing, and the forum would be overrun if it was present to any extent.
Did you change anything in your system recently; e.g., a Windows update? Are you using Windows 10 or earlier?
Have you tried deleting any Intuit-related cookies and deleted your browser cache to see if that makes a difference?
Are you using a wireless connection? If so, have you tried using a wired connection if available?
If you have another browser, it's a quick thing to try, but it might not help. If you don't have one easily handy, or if you already tried, try the following;
Try resetting everything in your Internet chain. i.e., shut down your computer, then router, and then modem. Wait a couple of minutes, then restart the modem. Wait a minute or so, then restart the router. Wait a minute or so and reboot (restart) your computer.
Here's how you can phone TurboTax Support tomorrow if you are using a paid product. Underneath your question above it says you are using TurboTax PLUS, which does qualify for phone support.
Support hours are 5AM-5PM Pacific (8AM-8PM Eastern), Mon-Fri
FAQ: What is the TurboTax phone number?
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
Or use this contact form to get a phone number:
https://support.turbotax.intuit.com/contact
